Chemistry of the Elements Flashcards
(357 cards)
1
Q
What are ores commonly found as?
A
Oxides
Sulphides
Carbonates
Silicates
Native elements
Noble metals
2
Q
How are elements from helium to carbon formed?
A
Fusion - e.g. three He nuclei fuse with a C nuclei and two gamma ray photons via a short lived intermediate beryllium nucleus
Process continues up to iron (most stable nucleus)
3
Q
How are heavier elements made?
A
Supernova nucleosynthesis
4
Q
How are elements beyond uranium formed?
A
Particle accelerators create new elements by bombarding a heavy element with highly accelerated lighter ions
5
Q
What are the main two types of bonds in the s and p blocks?
A
Ionic bonds - Positive and negative ions with no sharing of electrons
Covalent bonds - Sharing of electrons between atoms
6
Q
What gives a good indication of ionic or covalent bond character?
A
Difference in electronegativity:
Large difference is a good indication of charge separation and ionic bond character
Δχ>1.7
7
Q
How is the polarity of polar covalent bonds quantified?
A
Using dipole moments
μ = Qr (D)
8
Q
How do you find the degree of ionic bonding?
A
% ionic = (Q/e)*100 where e = charge of electron
9
Q
What does ionic contribution to bonding result in?
A
Bond strengthening and therefore a reduction in bond length
10
Q
When do you get the greatest overlap between orbitals?
A
When the orbitals are well matched in size and energy
11
Q
What happens to bond energies down a group and why?
A
Sigma and more predominantly pi bond energies decrease down the group
- Increasing valence orbital size
- Poorer overlap
- Weaker bonds
- Lower bond energies
12
Q
What is the overall trend in bond strengths with respect to orbitals?
A
2p-2p > 2p-3p > 3p-3p
13
Q
What is an oxidation state?
A
The apparent number of electrons added to or removed from an atom when it forms a compound
It is a formal assignment - may not reflect real nature of electrons
14
Q
How do you assign oxidation states for s/p compounds?
What is the sign of the oxidation state?
A
Assigned using electronegativities
Sign depends on atom it is bound to
15
Q
What is the oxidation state of elements?
A
Zero
Irrespective of atom (Ne), molecules (oxygen) or infinite lattice
16
Q
What compounds does hydrogen form?
A
Forms binary compounds (EHᵧ) with most elements
Falls into three classes
17
Q
What three classes do the compounds hydrogen form fall into?
A
Covalent (molecular) hydrides e.g. CH₄
Saline (ionic) hydrides e.g. LiH or CaH₂
Metallic hydrides e.g. LaH2.87
18
Q
What are things to know about covalent hydrides?
A
- Formed usually from p block elements
- Exist as individual discrete molecules
19
Q
What are things to know about ionic hydrides?
A
- Formed from most electropositive elements (typically s block)
20
Q
What are things to know about metallic hydrides?
A
- Formed with d/p block
- Typically non stoichiometric - non whole numbers in formula
- Electrically conducting solids with metallic lustre (appearance)
21
Q
What does the reactivity of a given EHx depend on?
A
Largely depends on electronegativities of E
22
Q
What happens if E and H have similar electronegativities in hydrides?
A
Homolytic bond cleavage occurs releasing radicals
E-H -> E· + H·
23
Q
What happens in compounds where E is more electronegative than H?
A
Heterolytic bond cleavage occurs releasing H+ (protic H atom)
- Behave as bronsted acids
24
Q
What happens in compounds where E is less electronegative than H?
A
Heterolytic bond cleavage occurs releasing H- (Hydridic H atom)
25
What is the trend in reactivity of hydrogen compounds to do with strength of E-H bonds in the s/p blocks?
Weakens going down the group
- Valence orbitals become larger/more diffuse
- Worse overlap
- Less stable so more reactive
Strengthens across a period
- Better overlap
- Increase in ionic character
- Tend to be more stable so more reactive
26
What are the alkali metals at ambient temp/pressure?
First 5 are metallic solids
Francium cannot be isolated
27
What is the reactivity of group 1 metals dominated by?
Ionisation energy where they get more reactive down the group
I.E. = -Eorb ∝ (Zeff/n)²
28
What packing structure do group 1 metals have and what happens to their properties down the group?
Body Centred Cubic (BCC)
Become softer and have lower melting point down the group
- Due to increasing s orbital size and metallic bonding
29
What compounds do group 1 metals form?
Chemistry dominated by the formation of M+ cations
Predominantly ionic in nature due to high electropositive nature
30
Why are the elements of group 1 highly reactive?
Charge felt by the outermost electron is low meaning it is readily ionised
This is due to this electron being in an ns orbital which is larger and more diffuse than other occupied AOs
31
What compounds can group 1 metals form? (actually)
Hydrides and halides
- All G1 metals react with these when heated
Hydroxides
- Metals and metal hydrides react violently with water
Oxides
- All G1 burn in air to form oxides
Carbonates
- Formed in many ways
32
What does the major product of combustion of G1 metals depend on and what are they?
Depend on the metal
- Li - oxide: O²⁻
- Na - peroxide: O₂²⁻
- K - superoxide: O₂⁻
33
What are the relative size of oxo-anions and metal cations of G1 and what does this mean?
Oxo-anions: O²⁻ < O₂⁻ < O₂²⁻
Metal cations: Li⁺ < Na⁺ < K⁺
Even though superoxide is smaller than peroxide it still bonds to Na as it can spread its charge across two sodiums
34
Are group 1 metals soluble in water?
Most G1 salts are soluble in water
35
How does solubility of salts change down G1?
For salts containing large anions - solubility decreases down the group
For salts containing small anions - solubility increases down the group
36
What does the trend in solubility of G1 metals depend on?
Lattice dissociation Gibbs energy of MX and Gibbs energy changes of hydration of the ions
37
How are lattice enthalpy of formation and dissociation linked?
Lattice enthalpy of formation = - Lattice enthalpy of dissociation
38
What is the equation to find solubility (ΔsolGmx)?
ΔsolGmx = ΔlattGmx(d) + ΔhydGm+ + ΔhydGx-
Solubility gibbs energy of MX = Lattice dissociation gibbs energy of MX + Hydration gibbs energy of M+ + Hydration gibbs energy of X-
39
When is a compound soluble?
Different size cation and anion
40
What happens if Δhyd dominates or Δlatt dominates?
If Δhyd dominates Δsol is negative and it is soluble
If Δlatt dominates Δsol is positive and it is insoluble
41
How do you improve the solubility of G1 in organic media?
To improve solubility of G1 complexes in non polar solvents it is possible to sequester (wrap up) the M+ ions using macrocyclic crown ether ligands
42
What happens to the crown ether ring size if there is a larger cation?
A bigger ring for larger cation
e.g. 12-crown-4 is for lithium but 18-crown-6 is for potassium
43
What are all the G2 metals at ambient temp/pressure?
What is special about radium isotopes?
All metallic solids
Radium isotopes are radioactive
half life = 1599 years
44
What happens to reactivity down group 2?
Reactivity increases down the group due to ionisation energy
However they are generally less reactive than G1 due to increase in Zeff
45
What is the trend of melting points in G2?
Typically decrease down the group however not nicely due to adopting different packing structures but decrease due to increase in size
Higher than G1 due to two e- participating in metallic bonding
46
What do group 2 metals not naturally occur as?
A free metal (M)
47
What compounds does group 2 form?
Oxides
- Burn in air to give oxides
Hydroxides
- Heavier members react with water to liberate hydrogen
Nitrides
Sulphides
Halogens
48
What should you know about reactions of group 2 to form oxides?
At ambient temp Be and Mg are passivated in this reaction and with water (kinetically inert)
- Due to protective oxide coating that forms on the metal surface - react slow enough
49
What should you know about reactions of group 2 to form hydroxides?
Only heavier members react with water
Can force Mg to react if treated with steam/hot water
50
Why is Be the odd one out?
It tends to form covalent compounds despite Mg, Ca, Sr and Ba forming predominately ionic compounds
51
Why does beryllium tend to form covalent compounds?
Be²⁺ has a very high charge density and is therefore able to polarise all anions and ligands to give polar covalent bonds
52
What happens to beryllium chloride in the vapour state?
BeCl₂ is monomeric and has a linear structure
It has 4e- so is electron deficient but is stabilised by van der Waals forces
53
What happens to beryllium chloride in the solid state and in solution?
Solid state: forms infinite chains with tetrahedral Be centres - has longer bond lengths than monomer due to electron donation (each Cl donates a lone pair into empty sp3 orbital on adjacent Be)
Solution: Lone pairs in solvents with donate instead
54
How are stable G2 coordination compounds formed?
Macrocyclic ligands can be used to form stable M²⁺ cation complexes
55
What are elements called that have properties between metals and non metals?
Metalloids
56
What happens to the metallic character around the p block?
Increases down a group (increase in n)
Decreases across a period (increase in Zeff)
Due to ionisation energies (low ie means higher metallic character
57
What happens to atomic radii of the p block elements?
Do not increase uniformly down groups due to d block and f block contractions
- Caused by inefficient shielding of Zeff by 3d and 4f electrons
58
What do many p block compounds act as?
Lewis acids or bases
The strength of the base depends partly on what orbital the lone pair is in
- Increase in s character = decrease in lewis basicity
59
What is a lewis acid or lewis base?
Lewis acid - electron pair acceptor
Lewis base - election pair donator
60
What compounds in the p block typically form lewis acids or lewis bases?
Group 13 - 6 valence e- - typically LA
Group 15-17 - all have lone pairs - typically LB
61
In group 14 and 15, what is there is difference between and why?
Difference between compounds of the second period and those of heavier elements
2nd period elements cannot expand their octet (steric clash)
Heavier elements can as they have relatively low lying empty d orbitals that can be used to accommodate more than 8 valence e-
62
What is group 18 known as?
What is group 17 known as?
What is group 16 known as?
What is group 15 known as?
What is group 14 known as?
What is group 13 known as?
What is group 2 known as?
What is group 1 known as?
The noble gases
The halogens
The chalcogens
The pnictogens
The carbon group
The boron group
The alkaline earths
The alkali metals
63
What is the max oxidation state in group 13? What else is there to know about group 13 oxidation states?
+3
This is the only important oxidation state for B and Al
Going down the group, +1 oxidation state becomes more significant
64
What is true about boron and elemental forms?
It has at least 5 covalent allotropes.
Amorphous elemental boron - brown powder
Crystalline form - shiny dark crystals
65
What is referred to as the inert pair effect?
Compounds having a lower oxidation state by 2 than expected
66
What is the inert pair effect?
As you go down G13
- The bond energies decrease due to poorer overlap of orbitals
- And the s electrons become more tightly bound (increasing promotional energy for hybridisation due to contractions of d/f)
- At bottom of group, energy required for promotion can be greater than energy released by formation of two new bonds -> transition to +1 oxidation state
67
How do you know whether G13 elements will form compounds of +3 or +1?
2 bond energies > promotional energy -> +3 oxidation state
2 bond energies < promotional energy -> +1 oxidation state
68
What are some G13 compounds that can be formed?
Trihalides
Boron-oxygen species
Boron hydrides
69
What are the different types of trihalides that G13 elements form?
Boron trihalides
Analogous aluminium trihalides
Heavier trihaldies
70
How are boron trihalides prepared and what do they look like?
Prepared by halogenation of boron oxides or by direct reactions with halogens
All monomeric with trigonal planar structure
71
What is true about B-X bonds in boron trihalides?
They are all shorter and stronger than expected which can be explained by looking at bonding present
- pi donation from F to B due to good overlap
- electron deficiency of B is partially relieved
72
Despite the extra stabilisation what are all boron trihalides?
Electron deficient so will act as lewis acids
- loses pi bonding while doing that as it makes sp3 hybridisation
- when determining lewis acidity of trihalides, pi donation and distortion go hand in hand
73
What are trihalides of heavier G13?
All lewis acids
- lewis acidity of the heavier trihalides reflects relative chemical hardness of the G13 element
74
What happens to the lewis acidities of G13 halides towards a hard LB or a soft LB?
Towards a hard LB, the lewis acidities of the halides weaken as softness of elements increase
BCl3 > AlCl3 > GaCl3
Towards a soft LB, the lewis acidities strengthen as softness of acceptor elements increase
GaX3 > AlX3 > BX3
75
What is HSAB theory?
'Hard' - small highly charged states, weakly polarisable species
'Soft' - big low charged states, strongly polarisable species
Hard acids tend to bind to hard bases
Soft acids tend to bind to soft bases
76
How do you form boric acid? (B(OH)₃)
BX3 compounds are all hydrolysed in water to give boric acid expect boron trifluoride
- The reaction stops at the adduct due to the high strength of the B-F bonds
77
What are the two types of bonds that exist in diborane?
'normal' 2c-2e bonds
'bridging' 3c-2e bonds
78
What happens to the oxidation states of the G14 elements?
Range from +4 to -4
+4 dominant for C, Si and Ge
+4 oxidation state becomes less prevalent down the group due to inert pair effect
79
What are the 5 carbon allotropes to be aware of?
Diamond
Graphite
Fullerenes
Graphene
Carbon nanotubes
80
What is special about the carbon allotropes diamond and graphite?
Diamond
- Tetrahedral sp3 with 4 sigma bonds per C
- Strong and dense
- High thermal conductivity/Electrical insulator
Graphite
- Trigonal planar sp2 with 3 sigma and 1 pi bond per C
- Alternate layers with strong intralayer and weak interlayer bonding
- Lubricant/Electrical conductor
81
What is the difference between the stability of diamond and graphite?
Diamond is metastable as graphite is thermodynamically more stable so conversion from diamond is thermodynamically stable
82
What is there to know about fullerenes?
C60, C70, C80
sp2 carbons in hexagonal and pentagonal rings
One carbon environment but two bond lengths due to localised single and double bonds
Localised C=C can undergo addition reactions
83
What is there to know about graphene and carbon nanotubes?
Graphene - 2D material consisting of a one atom thick layer of sp2 carbon atoms (single layer of graphite)
Carbon nanotubes - elongated cages of fused 6-membered rings
84
What is there to know about the heavier elements of group 14 surrounding structure?
Si and Ge adopt only diamond like structures
- multiple bonds are weaker down the group making graphite like structures unfavourable
Tin has two main polymorphs
- 6 coordinate metallic white tin
- 4 coordinate grey tin (formed by cooling white tin to 286K)
85
What are the general reactivity trends of G14?
Going down the group, the electropositive character and reactivity of the elements increase
86
What is catenation?
The ability of an element to form bonds with itself as part of a molecule?
87
What happens to catenation down G14?
Down group 14 catenation decreases due to poorer orbital overlap
88
What compounds do G14 elements form?
Oxides
EHₙ
Halides
89
What is there to know about G14 oxides?
Carbon forms two stable monomeric oxides, CO and CO₂
SiO₂ forms polymeric 3D solids in contrast to monomeric carbon dioxide
- Si double bonds are weaker due to poorer orbital overlap
- Needs to form four single bonds to be as stable
Dioxides of Ge, Sn and Pb are all non volatile solids
90
What is there to know about EHₙ compounds of G14?
Extensive chemistry of hydrocarbons
Heavier analogues exist and are prepared from the corresponding halides or oxides
- But less widely studied due to inherent instability
91
What is there to know about G14 halides?
Tetrahalides of G14 elements are all know exist lead iodide
All these halides are volatile covalent compounds
(Tin fluoride and lead fluoride are ionic)
Tetrahalides are all lewis acids and use low lying d orbitals to accept electrons (except carbon)
92
What is nitrogen?
A very stable diatomic gas with a high N triple bond dissociation energy
- Also kinetically inert due to lack of dipole moment
- N-N bonds are weak due to lone pair repulsions
Due to small size N can only form up to 4 bonds
93
What are the three main allotropes of phosphorus?
White - tetrahedral P₄ molecules
- very reactive due to lots of bond strain and accessible lone pairs
Black - interlinked 6 membered rings of 3 coordinate P
- least reactive and most thermo stable
- obtained by heating white under pressure
Red - several crystalline forms
- Intermediate stability
94
Why does white phosphorus have lots of ring strain?
They are at 60 degrees compared to preferred 107 degree angle
95
What is there to know about As, Sb and Bi?
Grey solids under standard conditions
Have lattice structures resembling black phosphorus
Combine readily with oxygen
96
What oxidation states do G15 elements have in compounds?
+3 or +5 in covalent compounds
97
What happens to oxidation states down G15?
+3 oxidation state becomes more stable due to inert pair effect except trend is less well defined than in G13-14
98
What can compounds in G15 act as?
Lewis acids or bases (R₃E)
- Are all lewis bases due to presence of lone pair
- Can also act as lewis acids due to vacant low lying d orbitals
99
What does lewis acidity and basicity of G15 compounds depend on?
Primarily on the electronegativity of the R groups
Basicity - higher the e-neg of R, lower the lewis basicity of compound
Acidity - higher the e-neg of R, higher the lewis acidity of compound
100
What compounds does G15 form?
Hydrides
Halides
Oxides
101
What is there to know about G15 trihydrides?
Relating to reactivity and boiling points
Trihalides become more reactive and less stable down group (weak bonds due to orbital size mismatch)
Boiling points vary irregularly due to hydrogen bonding down the group (we expect increasing size -> increasing vdW -> increasing b.p.)
NH3 >> PH3 < AsH3 < SbH3
102
What is there to know about G15 halides?
All G15 elements form trihalides and from phosphorus down they form pentahalides
103
What is the difference between trihalides of nitrogen and trihalides of phosphorus?
Trihalides of nitrogen are all extremely unstable - instability reflects the thermodynamics of decomposition (formation of N₂ is often driving force)
Trihalides of phosphorus are all stable and are mainly formed by direct reaction of P and halides
- Fluorine however needs halogen-exchange
104
What happens to the halides of G15 down the group?
Get more ionic
- PX5 species very much on border between ionic and covalent
- PCl5 exists as a liquid and gas but in solid state it exists as PCl6- or Pcl4+
105
What is there to know about G15 oxides?
Nitrogen displays a wide range of oxidation numbers in its oxides (+1 to +5)
As you go down group - decreasing stability of multiple bonds to oxygen
- Results in no comparable oxides from other elements except P
106
Why is P=O stable?
Stability of P=O is due to strong ionic component and empty d orbitals that can accept electron density from oxygen
107
What is the valence electron config for G16 elements?
What is therefore the highest oxidation state and range of oxidation states of oxygen?
ns2np4
Highest = +6
Oxygen ranges from -2 to +2
108
What happens to metallic character down G16?
Metallic character increases
O/S - Non metals
Se/Te - Metalloids
Po - Metal
109
What is there to know about the most common allotrope of oxygen in G16, dioxygen (O2)?
Paramagnetic as it contains two unpaired electrons in its pi* orbital
Very powerful oxidation but kinetic barrier to reaction is high
- At high temps it will combine with most elements
Gas is colourless and liquid is pale blue
110
What is another allotrope of oxygen?
Ozone O3
A bent, diamagnetic, triatomic molecule
Created in nature by
- Electrical discharges from thunderstorms reacting with O2
- Action of UV radiation on O2
In each process O atoms are formed that then react with O2 to form ozone
111
What is the preference of catenation in G16?
O < S > Se > Te > Po
Sulphur tends to catenate
112
What compounds do G16 elements make?
Hydrides
Halides
Oxides
113
What oxidation states do all chalcogens exhibit?
-2, 0 and +2 oxidation states where all except oxidation commonly exhibit +4 and +6
114
When does the -2 oxidation state of G16 elements occur?
Commonly in compounds with electropositive metals
115
Why does oxygen form the widest range of ionic species?
Possesses the highest electron affinity of G16
- Oxygen is also present in lots of compounds containing strong multiple bonds
- Multiple bonding less significant down group
116
Why is there more known ionic oxides than ionic nitrides?
O is more e-neg than N and hence has a higher tendency to form negative ions
Oxygen bond dissociation energy is a lot less than that of nitrogen
117
What are the hydrides of oxygen?
Water - bent molecule 105 degrees - forms from direct reaction of its elements
Hydrogen peroxide - strong oxidising agent
- Susceptible to decomposition by disproportionation
118
What is true about heavier hydrides and why?
Also bent like oxygen hydrides but with bond angles of around 90 degrees
- E-H bonds have significant p orbital character from central atom and little to no contribution from valence s orbital (not fully sp3 hybridised)
119
What are the boiling points of G16 hydrides?
Boiling points: increasing vdW between larger molecules increases b.p.
- Strong H bonding is reason for trend
H2O > H2S < H2Se < H2Te
120
What is the thermal stability of G16 hydrides?
Down the group the bonds to H get weaker due to poorer orbital overlap
H20 > H2S > H2Se > H2Te
121
What is the acidity of G16 hydrides?
The weaker X-H bonds increasingly favour the equilibrium so increase acidity
H20 < H2S < H2Se < H2Te
122
What happens with oxygen halides?
Oxygen forms several halogen oxides and oxo-anions
Oxidation state of -2 except for fluorine (+2)
123
What happens when sulphur reacts with fluorine?
Gives the gas SF6
- Very inert due to: Strong bonds, sulphurs steric protection and all of sulphurs available orbitals being used in bonding
124
What is true about SF6 and how does it compare to SF4?
SF6 stable in water and air - only attacked at high temp
Gaseous SF4 with even stronger bonds is extremely reactive to water
125
How can the difference in reactivity be explained between SF6 and SF4?
Varying structures
SF4 has an open face which is easy to attack
126
What oxides of G16 compounds are formed?
Most important oxides of sulphur are SO2 and SO3
Oxo-anions [SO₃]²⁻ and [SO₄]²⁻ are prepared from SO2 and SO3 respectively by reaction with water in acidic conditions
127
What is the difference between sulphites and sulphates?
Sulphites are highly reactive due to lone pair and open structure
Sulphates very inert due to tetrahedral array of oxygen providing kinetic stability
128
What is the valence electron config of G17 and what does that mean for oxidation states?
ns2np5
Highest possible oxidation state of +7
129
What is key to know about oxidation states of G17 elements?
All halogens exhibit -1, 0, +1, +3, +5 and +7
Except for fluorine - only -1 and 0
130
What happens to boiling point of G17?
Increases uniformly down the group
Due to large atoms so greater VdW interactions
131
What happens to thermal stability of G17?
Decreases down the group
Except for F2
- Smaller atom so increased lone pair repulsions (compared to N and O)
- High reactivity as F-F bond energy much less than Cl-Cl
- Also ability to form strong bonds influences reactivity
132
What are the colours of dihalides and why are they seen?
Can be explained using MO theory where an electron from HOMO is excited to LUMO
Flourine - pale yellow
Chlorine - green/yellow
Bromine - red/brown
Iodine - violet
133
What does the HOMO-LUMO gap in dihalides give us?
The colour of the dihalogen
- Gap decreases as group is descended
- Based on similarity of size of AOs
134
What basic compounds do G17 elements form?
Hydrides
Oxides and oxo-acids
135
What is there to know about hydrides of G17?
Thermal stabilities and bond strengths decrease down group
All are gases at room temp
All acids in water
136
Why is HF a much weaker acid than all other hydrogen halides?
Due to very strong H-F bonds and unfavourable loss of these H bonds upon ionisation
- At high HF concs, there is an increase in acidity due to extra stabilisation of F- (formation of [HF₂]⁻ anion)
137
What is there to know about oxides and oxo-acids of G17?
All oxides of Cl, Br and I are thermodynamically unstable except I₂O₅ (decompose explosively)
- Weakness of X-O bonds means extremely powerful oxidants
Forms 4 types of oxo-acids from Cl, Br and I
138
What is there to know about molecular fluorides?
Mean bond enthalpies for X-F bonds are particularly large so a wide range of molecular fluorides are known
- Fluorine is effective at bringing out highest valency of non metals and highest oxidation state of metals
- Covalent fluorides often volatile as F very unpolarisable so weak vdW leading to low b.p.
139
What is there to know about (per)chlorate anions?
Perchlorate anion (ClO₄⁻) less reactive than chlorate (ClO₃⁻)
- More oxygen makes it more stable (kinetic reasons)
At high temps kinetic barrier can be overcome and perchlorate are extremely powerful oxidants
Chlorates are also strong oxidising agents
140
What is there to know about interhalogen compounds?
Mixture of halogens
- Have general formula XYₙ where n is odd
- Can rationalise shape using VSEPR
Can all be prepared by direct combo of elements
141
What is true about the noble gases?
All have closed shell electronic configs
All mono-atomic gases at room temp
Generally unreactive and form a limited range of compounds
142
What happens to the boiling points of noble gases?
Increase down the group with increasing atomic weight
143
What compounds do G18 elements form?
Fluorides and oxides
144
What is there to know about G18 fluorides?
Progressive fluorination - XeF2, XeF4, XeF6
Xe-F bonds have high ionic character and large bond dissociation enthalpies
- compensates the large I.E. of xenon
Strongly oxidising and fluorinating agents
Reactivity: XeF6 > XeF4 > XeF2
145
What is there to know about G18 oxides?
Xenon oxides are unstable and highly explosive
Prepared from hydrolysis of xenon hexafluorides
146
What are isoelectronic molecules?
Species that contain the same number of valence electrons
e.g. N2, CO, NO+
147
Are isoelectronic species also isostructural?
Often they are
However not always the case
e.g. CO2 and SiO2
148
What is there to know about BN compounds?
BN unit is isoelectronic with C2 - strong structural relationship between BN and C compounds
However doesn't mimic CC unit chemically due to e-neg values (dipole vs no dipole)
149
How do benzene and borazine compare?
Borazine is isoelectronic with benzene and similar electronic structure
Can be prepared from (NH4)Cl and Na(BH4)
Its reactivity is very different than benzene due to polarity of BN bond
150
How do graphite and boron nitride (BNx) compare?
Boron nitride - robust and rather chemically inert
- Can be prepared from high temp reactions
Has a similar structure to graphite
- Lots of B-N dipole interlayer interactions so less easy to cleave
- Polar B-N bonds (less delocalisation) so electrical insulator whereas graphite is a conductor
151
What are diagonal relationships in the periodic table?
Observed between period 2 and 3
Similar chemical/physical properties as a consequence of similar size and e-neg
152
What are coordination compounds?
Central metal ion M surrounded by a shell of ions or molecules L
Normally M = e- acceptor or lewis acid and L = nucleophile or lewis base
153
How many ligands can you have in coordination compounds?
Any number from 1-12
154
What are the types of coordinative bond?
A line = interaction between anionic ligand and an acceptor
An arrow = donation of e- from a donor to an acceptor
Often just a single line used
155
What are ligands?
Charged or neutral atoms, ions or molecules that can form coordinative or dative bonds to metal centres. They possess lone pairs
156
What are the types of ligand?
sigma donor ligand - donating e- den often using s-orbitals or sp hybrids
(sigma acceptor ligand - accepting e- den using s orbitals)
pi donor ligand - able to donate e- den using p orbitals
pi acceptor ligand - able to accept e-den using p orbitals
157
What do pi donors and pi acceptors do?
Combine sigma and pi components (this is what forms the bond)
158
What is hapticity?
Hapticity ('eta' η) - number of neighbouring atoms in a ligand that are coordinated simultaneously to a metal centre
159
What is denticity?
Denticity ('kappa' κ) - number of non-neighbouring atoms in a ligand that are coordinated simultaneously to a metal centre
160
What are the formation (stability) constants (Kf)?
They quantify bond formation, especially in ligand substitution processes
'Better ligand will want to be bonded to central metal’
161
What do formation constants allow for?
Allow calculations of thermodynamic values
ΔG = -RTln(K) = ΔH - TΔS
162
What are stepwise formation constants? How can they let you calculate the overall formation constant?
Formation constants for the addition of each ligand to the metal centre, K1, K2 etc
Overall formation constant: βₙ = K₁ * K₂ * ... * Kₙ = [MLₙ]/[M][Lₙ]
Usually expressed as log βₙ =
163
What happens to values of formation constants with each successive substitution?
Typically decrease
- If not likely a change in geometry
164
What factors affect complex stability (monodentate L only)?
Ionic size and charge
Hard and soft metal centres and ligands
165
How does ionic size and charge affect complex stability?
Stability of metal ions of a given charge decrease with increasing cation size
Similar behaviour for s,p and f blocks but not so simple for transition metals
166
How does hard and soft metal centres and ligands affect complex stability?
Hard cations form more stable complexes with hard ligands (ionic bonding and entropy driven)
Soft cations form more stable complexes with soft ligands (covalent bonds and enthalpy driven)
Hard soft not favoured as not enough energy is released to overcome high solvation energy of hard species
167
What happens with increased denticity and what are the names given dependent on the number of donor atoms?
Increased denticity = increased binding strength
1 - monodentate
2 - bidentate
3 - tridentate
4 - tetradentate
5 - pentadentate
6 - hexadentate
168
What is a chelate?
A complex formed when a single ligand binds to a metal through two or more donor atoms
169
What is the chelate effect?
Confers increased stability on a complex
Consisting of:
- probability (polydentate less likely to be displaced than monodentate)
- sterics (chelate often prevent attack)
- entropy
170
How is entropy so important in the chelate effect?
If number of molecules increases with ligand substitution there is increased stability compared to non-chelated anologues
171
What is the difference between meridional (mer-) and facial (fac-)?
Facial - ligand donors form a triangle on one face
Meridional - ligands donors span a meridian, forming a linear like relationship - essentially in a line
172
What are the different types of tetradentate ligands?
Open chain (unbranched)
Macrocyclic
Tripodal - three branches (like a tripod)
173
What is there to know about EDTA?
It binds extremely well to first row TMs
Displaces monodentate ligands, forming stable chelates where the metal ion is encapsulated
Can bind in a pentadentate fashion or as a hexdentate ligand
174
What are terminal/bridging (μ) ligands?
Terminal ligands - directly bonded to only one metal centre
Bridging ligands - connects two or more metal centres creating a bridge
175
What are ambidentate ligands?
Ligands capable of donating electrons from more than one donor site
- different metals bind differently based on hard/soft acid/base
176
What is true about electron configurations of d block elements?
'Normal' electron configurations only hold for gaseous neutral atoms of d block elements
In metal complexes, the d orbitals are stabilised to a much higher degree so 4s,5s and 6s orbitals are not occupied (unless d orbitals are full)
177
How do you calculate the d occupancy for d block complexes?
d-occupancy = group number - oxidation state
178
What is an important aspect of d-block complexes?
Ability to display different oxidation states, key in catalysis
179
What is the Kepert model?
VSEPR is not applicable to d-block metals
The kepert model rationalises the shape of d-block metal complexes by considering the repulsion between ligands (lone pairs are ignored)
180
What are some less common geometries you may need to know for coordination compounds?
Trigonal pyramidal - CN = 3
Trigonal prismatic - CN = 6
181
What is the difference between a association and dissociation reaction?
Association - occurs when a ligand binds to a metal centre - increasing CN if metal is not fully coordinated
Dissociation - reverse process - a ligand detaches from a metal centre decreases CN
182
What is there to know about CN 1/2 and 3?
CN 1 - Requires sterically very bulky ligands
CN 2 - Common in metals of G11/12 (all linear)
Cn 3 - Not a very common geometry - Often involve very bulky ligands
183
What is there to know about CN 4?
Favoured over higher CN if the central atom is small or ligands are large
Square planar is less common than tetrahedral and is mainly associated with d8 metal ions
184
What is there to know about CN 5?
Quite common and both trigonal bipyramidal and square based pyramidal are both adopted (sometimes by the same compound)
- Very small energy difference between the geometries
185
What is berry pseudorotation?
Occurs in both transition metal complexes and also main group compounds
The ligands exchange between axial and equatorial sites (fluxionality), leading to two environments being indistinguishable by many spectroscopic methods
186
What is there to know about CN 6?
Octahedral is extremely common and most important geometry for d1 to d9 complexes
Trigonal prismatic is extremely rare but a few examples do exist (often d0 complexes)
187
What can happen to octahedral coordination complexes?
Distortions can occur for MX5Y, MX4Y2 and MX3Y3
188
What are the three different types of complex?
Neutral
Anionic
Cationic
All of the complex is written in square brackets with any balancing anions/cations outside the brackets
189
What is the difference between inner and outer sphere complexes?
Outer: anion bound electrostatically
Inner: anion as a ligand bound directly to the metal centre (primary coordination sphere)
190
What is meant by nuclearity when it comes to describing complexes?
Number of metal centres in a complex
Can be
- mononuclear
- dinuclear
- polynuclear (clusters)
191
What does everything that is not mononuclear contain?
Bridging and/or metal metal bonds
192
What is the sequence of central atom and ligand formulae?
Square brackets indicate ligands attached to M
Central atom first followed by hydride if present followed by negatively charged ligands and finally neutral ligands
- Ligands ordered alphabetically within negative or neutral
Charge written as number in superscript
193
What is the sequence of central atom and ligand names?
Ligands listed alphabetically regardless of charge and prefixes
Central atom placed after the ligands
Oxidation state in parenthesis
194
How do you assign the number of ligands in a complex?
There is two types of numerical prefixes:
i) di, tri, tetra, penta ...
ii) bis, tris, tetrakis, pentakis
i) is recommended, but ii) is used for complicated ligands and when there could be ambiguity
195
What is important to note when naming anionic complexes?
They end in 'ate'
e.g. is central metal is iron then name in anionic complex is ferrate
196
What is important to note when thinking about oxidation numbers and charge numbers when naming complexes?
Oxidation state number if known is indicated as a roman numeral
Charge numbers are indicated by arabic numerals preceding the charge sign (e.g. 4-)
197
What is the difference between constitutional isomers and stereoisomers?
Constitutional isomers - same empirical formula but different atom connectivity
Stereoisomers - same connectivity but individual atoms are arranged differently in space
198
What is linkage isomerism?
Where the donor atom of at least one of the ligands is different
Needs ambidentate ligands
199
What is ionisation isomerism?
Occurs when isomers produce different ions in solution, often by the exchange of a ligated anion with a counter ion
(includes hydrate isomerism)
200
What is coordination isomerism?
When ligands are distributed differently between two metal centres
201
What is ligand isomerism?
Ligands in different isomeric forms e.g. ortho, meta, para forms
202
What is geometrical isomerism?
cis and trans isomerism that can occur in square planar, trigonal bipyramidal and octahedral
- square planar - two isomers
- trigonal bipyramidal - three isomers
- octahedral - cis/trans and mer/fac
203
How can you predict which geometrical isomer will form?
The size of the ligands should be considered
Large ligands favour the trans isomer on steric grounds
204
What is conformational isomerism? (or polytopal isomerism) And when can it occur?
Interconversion of two geometries
- Possible with any CN for which there is more than one known stereochemistry
- Isomers must be of comparable stability and possess a significant energy barrier preventing interconversion
205
What is optical isomerism?
Isomeric chiral complexes that are mirror images of each other
- Non superimposable mirror images
- A common isomerism in octahedral compounds with chelating ligands
206
What is Crystal Field Theory (CFT)?
A purely ionic description of bonding in TM complexes
Predicts the 'splitting' of d-orbitals in complexes with defined geometries
207
What are the assumptions made for CFT?
- Metal centre and ligands are point charges
- Bonding arises through electrostatic interaction between these charges
208
What is true about the d-orbitals in free metal ions?
All 5 are degenerate
209
Where do the d orbitals lie?
dx2-y2 lies on x and y axes
dz2 lies on z axis
dxy, dxd, dyz lie in the plane of, and in between the labelled axes
210
What is true about the uniform spherical ligand field?
All d-orbitals are still degenerate but now higher in energy due to repulsion
211
In octahedral complexes, which d-orbitals point directly towards the ligands?
dx2-y2 and dz2 point directly towards the ligands
dxy, dxz and dyz point between the ligands
212
What happens to d-orbitals that point directly towards or between ligands?
d-orbitals pointing directly towards ligands are raised in energy due to electronic repulsion
d-orbitals pointing between the ligands are lowered in energy due to electronic relief
213
What are the labels for the raised/lowered d-orbitals in octahedral complexes?
Raised - eg (subscript g)
Lowered - t2g (subscript 2g)
t stands for triple
214
What is the result of raising and lowering of d-orbital energies?
Splitting of the d-orbitals into two sets
215
What is the difference between the two energies of d-orbitals in octahedral complexes?
Crystal Field Splitting Parameter (CFSP) - Δoct
216
What is the barycentre?
The energy if the ligands were presumed to be a uniform spherical field
217
How much are each set of orbitals in octahedral complexes raised and lowered by relative to Δoct?
eg is raised in energy by +3/5Δoct
t2g is lowered in energy by -2/5Δoct
218
How do we know how many electrons to place in splitting diagrams?
Using the dⁿ count
219
What is the Crystal Field Stabilisation Energy?
Energy to which the complex is stabilised for having a particular arrangement of ligands compared to the spherical ligand field situation
220
How do you calculate the CFSEoct?
((-2/5 * no of e- in t2g)+(+3/5 * no of e- in eg))Δoct + zP
221
What does the P stand for in the formula for CFSE and why is it included?
P = Pairing energy
Pairing electrons requires energy (electronic repulsion) so there is an energy penalty
222
What is the first complex that we need to consider high/low spin for?
d4 complexes
223
How do you distinguish between high spin and low spin?
High spin has more unpaired electrons than low spin
High spin - each orbital is filled by one electron before electrons are paired
224
What are P and Δoct dependent on?
P is dependent on a given metal and its oxidation state
Δoct is dependent on metal, oxidation state and ligand
225
How can you calculate whether a complex will be high spin or low spin?
If Δoct < P then high spin (more favourable to put electron in orbital with higher energy than to pair)
If Δoct > P then low spin (more favourable to pair electrons than to put electron in orbital with higher energy
226
How can you tell field strength from high spin and low spin?
High spin means weak field
Low spin means strong field
227
By calculating Δoct of each configuration (high and low spin) how do you know which configuration is used?
The configuration with the lower CFSEoct is used
228
What is key not to forget when calculating CFSE?
The pairing energy P for both the ligand field and the uniform spherical ligand field
- Pairing in that needs to be accounted for aswell and it is taken away from overall energy
229
What is true for d8-d10 complexes? (and for d1-d3) for octahedral complexes
There is no high/low spin configuration (both would end up with same placement of electrons)
230
Which complexes and their configurations have 0 CFSEoct?
d10 and high spin d5
231
What factors affect the size of Δ in orbital splitting diagrams?
Depend on both the metal and the ligands
- Metal oxidation state
- Metal position on periodic table
- Ligands
232
How does Δ vary with oxidation state of the central metal?
Δ increases with increased oxidation state of central metal
- Increased oxidation state pulls ligands closer due to increased Zeff
- Larger electrostatic repulsion between e- in d orbitals and ligands
- Increase in splitting
233
How does Δ vary with position of the metal centre in the periodic table?
Δ increases down group of metals in the periodic table
- d orbitals get bigger which means two things
234
If the size of the d orbitals increase what does this mean in coordination complexes?
1) better contact with ligands, more interactions, more destabilising, increase Δ
2) pairing energy decreases as more room to pair electrons, less repulsive so smaller energy penalty
235
What is an important consequence of d-orbitals getting bigger down a group for TM complexes?
All 2nd and 3rd row octahedral TM complexes are low spin
236
How will the ligands in a complex vary and how can we rank ligands in terms of the effect they have on Δ?
Will vary in the nature of bonding with the metal
Can rank them using the 'Spectrochemical Series' which is derived from experiment
237
How does the Spectrochemical Series order ligands?
In terms of increasing 'strength' or increasing Δ splitting
238
What is true when Δoct is small/large?
If Δoct is small then weak field and high spin
If Δoct is large then strong field and low spin
239
What does the general trend of the spectrochemical series follow?
The strength of sigma donation - ligands that form strong sigma bonds cause larger Δ (more interaction)
240
What is the specific trend of the spectrochemical series dominated by?
pi effects not sigma effects
241
What donor atoms are poor/good sigma donors?
E-neg donor atoms are poor sigma donors as they try to keep their electrons and not donate
Less e-neg donor atoms are better sigma donors as don't want the negative charge
242
What is the trend of the spectrochemical series based on pi effects?
strong pi-donor < weaker pi-donor < no pi-donation < weaker pi-acceptor < strong pi-acceptor
243
What is true about strong pi donors and acceptors?
Strong pi donors cause smaller d-orbital splitting whereas strong pi acceptors cause increased d-orbital splitting
- Failure of crystal field theory as need covalent arguments to justify trends in Δ
244
What are pi donors and pi acceptors in CFT?
Pi donors - have lone pairs available in appropriate orbitals that can be donated to empty d orbitals on the metal
Pi acceptors - have empty orbitals of appropriate symmetry that can receive electron density from metal d orbitals
245
How to know if a ligand may be a pi-donor or pi-acceptor?
Extra lone pairs could be a pi donor
If has a pi bond it could be a pi acceptor
246
What is the alternate axis that we need to define for tetrahedral CFT?
All four ligand positions are equivalent
Place the ligands on alternate corners of a cube
- This allows us to define an axis where x,y and z are centred on the metal ion and each axis cuts through the centre of each face of the cube
247
How else can you view the axis system for tetrahedral CFT?
As a 'plan' so flat
Looking along the z axis you will get a square of ligands in each quadrant of xy axes with two just below plane and two just above plane
248
What is true about where the d orbitals point in octahedral CFT than in tetrahedral CFT?
In tetrahedral CFT none of the d-orbitals point directly towards the ligands so won't have the same extent of splitting
249
What happens, specifically in tetrahedral complexes, when d-orbitals point different amounts towards the ligands?
Those pointing more towards the ligands are raised in energy and destabilised due to electronic repulsion
Those pointing between the ligands are lowered in energy and stabilised due to electronic relief
250
What are the labels for the raised/lowered d-orbitals in tetrahedral complexes?
Raised - t2 (2 in subscript)
Lowered - e
No g as no inversion centre
251
What d orbitals are raised/lowered in energy in tetrahedral complexes?
dxy, dxz and dyz are raised
dx2-y2 and dz2 are lowered
252
What d orbitals are raised/lowered in energy in octahedral complexes?
dx2-y2 and dz2 are raised
dxy, dxz and dyz are lowered
253
What is the difference between the two energies termed in tetrahedral complexes?
Crystal Field Splitting Parameter, Δtet
254
What is the energy, relative to the barycentre, of the two new sets of orbitals in tetrahedral CFT?
t2 is raised in energy by 2/5Δtet
e is lowered in energy by 3/5Δtet
255
How do you calculate CFSEtet?
CFSEtet = ((-3/5 * no of e- in e) + (2/5 * no of e- in t2))Δtet + zP
256
What is the important result comparing Δtet and Δoct?
Δtet is always small compared to Δoct
Δtet = 4/9Δoct
257
What is the consequence of the small value of Δtet?
Δtet is always lower than the pairing energy P so all tetrahedral complexes are high spin
258
What does tetrahedral deal better with over square planar?
Sterics due to larger bond angles but not necessarily electronics
259
Why does the square planar splitting diagram change relative to the octahedral one?
d orbitals with components parallel to z axis stabilise as no ligands in that direction (less repulsion)
And to keep the energy balance other orbitals must be raised in energy
260
What can you assume for the spin of square planar complexes?
That they are all effectively low spin due to the large splitting between the d-orbitals
261
How do we form a square planar complex from a octahedral complex?
Removing the ligands that sit on the z axis
262
What d-orbitals are stabilised and destabilised?
dz2, dxz and dyz are stabilised and lowered
dx2-y2 and dxy are destabilised and raised
263
What is the order of relative energy of the d-orbitals in square planar complexes?
dxz and dyz degenerate
dz2
barycentre
dxy
dx2-y2
Increase in energy down the list
Order for tetragonally distorted ligand field:
dxz and dyz degenerate
dxy
dz2
dx2-y2
264
When should you consider square planar geometries?
If the complex has coordination number 4 and has a d8 configuration it will be square planar if:
- Metal is 3d8 and has strong field ligands
- Metal is 4d8 and 5d8 with any ligand
265
Why if the metal is 3d8 do you need strong field ligands for a square planar complex to appear?
Strong field ligands give larger Δ so promotes square planar arrangement which has larger splitting compared to tetrahedral
266
Why if the metal is 4d8 or 5d8 can you have any ligand for square planar complexes?
2nd and 3rd row TMs have larger d-orbitals, more ligand interaction which gives larger Δ and favours square planar
267
What type of ligand might force a certain geometry?
So bulky or constrained so force a certain geometry
e.g. macrocycles can force square planar and bulky bidentate ligands can force tetrahedral
268
What is the simpler translation of the Jahn-Teller effect for transition metal complexes?
A system will remove the degeneracy of orbitals and undergo distortion to remove the 'ambiguity' of which orbital an electron will occupy (overall more energetically favourable)
269
What will the Jahn-Teller effect do to the complex?
The complex will commit to splitting the d-orbitals further to place electrons in specific d-orbitals to then have a consistent (but distorted) structure
270
Where is the distortion caused by the Jahn Teller effect most often observed?
In octahedral complexes when there is an odd number of electrons in the eg (destabilised) orbitals
271
What are the two potential distortions from the Jahn Teller effect?
As the 'ambiguous' electron can be in one of the dx2-y2 or dz2 orbitals there can be:
- z-elongation and xy compression
- z-compression and xy elongation
272
What happens to orbitals in z-compression in octahedral complexes?
dxy goes down a tiny bit and dxz and dyz go up a tiny bit
dx2-y2 goes down a tiny bit and dz2 goes up a tiny bit
- no longer ambiguity
273
What happens during z-elongation?
z axis ligands move further away from the metal (as xy ligands compress). This lessens repulsion between ligands and metal d-orbitals that have a z component and increases repulsion between those that have an x and y component
274
What happens during z-compression?
z axis ligands move inwards towards the metal (as xy ligands move outwards). This increases repulsion between ligands and metal d-orbitals that have a z component and reduces repulsion between those that have an x and y component
275
What is octahedral site preference energy?
It is the preference a complex has to form an octahedral complex, rather than a tetrahedral complex
276
How do you calculate octahedral site preference energy?
OSPE = CFSEoct - CFSEtet (high spin)
OSPE is large for d3 and d8 - strong preference for octahedral complexes
No preference for oct/tet for d5 and d10 as these have 0 CFSE
277
What, related to CFSEoct, is reflected in hydration energies?
The 'double hump'
It deviates from expected trend if only based on Zeff due to extra stabilisation for CFSE
278
What does it show us about hydration energies that d0, d5 and d10 have no CFSEoct?
They are effectively just hard spheres so are on trend
279
What is the expected trend in hydration energies?
Decrease across a period if only based on Zeff
280
What is the colour wheel?
Red, Orange, Yellow, Green, Blue, Indigo, Violet
Red, Orange, Yellow, Green, Blue, Purple
281
How do colours in metal complexes arise?
Primarly from two types of transitions
- Transitions between d-type orbitals (d->d transitions)
- Charge transfer (CT) between metal and ligands
282
What type of spectroscopy is electronic absorption spectroscopy?
UV-vis
283
Where do transitions need to occur for colour to be seen?
Need to occur within the visible region for colour to be seen
284
How do you convert between kJ/mol to cm-1?
What about between nm and cm?
1 kJ/mol = 84 cm-1
x nm = 10^7/x cm-1
285
What happens in a d to d transition of metal complexes?
As d orbitals are not degenerate electrons can be excited from a lower energy orbital to a higher energy orbital
286
How do d-d transitions occur?
White light is shone onto complex which excites an electron
287
How do you know what colour a d to d transition occurs?
Find Δoct between the d-orbitals
This is where it absorbs
Convert it to nm and see what the value is
Opposite colour in the spectrum is what the colour of the complex is
288
Why should caution be exercised when there is more than one absorption that contributes to the colour?
Could be to do with other d-d transitions, the should of a charge transfer band encroaching on the visible region, or colour due to ligands
289
Roughly what are the nm/cm-1 values at the extremes of the visible region?
700-380 nm
14300-26300 cm-1
290
How do we see liquid colour vs solid colour?
Liquid colour = absorption of light
Solid colour = reflection of light
291
What effect does oxidation state have on the magnitude of Δ and therefore on colour?
Increase in oxidation state - Greater attraction between metal and ligands - Larger Δoct - High energy - Shorter wavelength light absorbed
292
What effect do ligands have on the magnitude of Δ and therefore on colour?
Different ligands cause different extents of spliting
Addition of a weak field ligand - Smaller Δoct - Lower energy - Higher wavelength light absorbed
293
What is important to remember about the colour observed and the intensity?
The colour observed depends on the light absorbed whereas the intensity depends on how well the transition obeys the selection rules
294
How do we get the colours black and white?
Black - absorbing across vis region
White - not absorbing in vis region
295
For transitions in metal complexes to occur what selection rules must be obeyed?
Spin selection rule: Transitions between states of different spin are forbidden (ΔS=0)
Laporte (Parity) selection rule: Transitions must occur between states of different parity
296
What does the Laporte selection rule mean?
g <-> u transitions are allowed but g <-> g and u <-> u are forbidden
297
What does the Laporte selection rule lead to?
Leads to the associated selection rule: Δl = ±1 where l is orbital angular momentum quantum number
- Meaning e.g. s <-> p are allowed but s <-> are forbidden
298
Due to the existence of the selection rules what are d-d transitions?
Many d-d transitions are relatively weak
299
What does the Laporte selection rule apply to?
Only centrosymmetric metal complexes
300
When can promotion of an electron proceed? (spin selection rule)
If spin orientation is preserved
301
What happens in octahedral complexes with six of the same ligand?
d-d transitions are all g-g so are all forbidden
302
What happens to the Laporte selection rule in tetrahedral complexes?
They are not centrosymmetric so more p-d mixing can occur, meaning their d-d transitions don't need to obey this selection rule
303
As a result of not needing to obey the Laporte selection rule, what happens to the intensity?
Intensity ε (Tet) is 100x greater than intensity ε (Oct)
304
When can selection rules be broken?
Vibronic coupling
Spin-orbit coupling
Mixing of MOs (through pi bonding) so transitions no longer purely d-d
305
How does vibronic coupling break selection rules?
It removes symmetry, leading to p-d mixing so d-d transitions are allowed while in distorted geometry
(Jahn-Teller distortions also break the symmetry and allow relaxation of Laporte selection rule)
306
What is a charge transfer absorption and how strong are they?
Movement of electronic charge between metal and ligand
Often 1000 times stronger than d-d transition bands
307
What is Ligand to Metal CT (LMCT)?
Transfer of e- from p-orbital in ligand (u) to a metal d-orbital (g)
- Favourable for high oxidation state of metal
- Common for pi donors
308
What is Metal to Ligand CT (MLCT)?
Transfer of e- from metal d-orbital to pi* orbital of ligand
- Favoured for electron rich metal centres with pi-acceptor ligands
- Often higher in energy (in the UV)
309
What happens when an isolated atom is placed in a magnetic field?
Interaction because each e- in an atom acts like a magnet
- Electron has a magnetic moment
310
What is a magnetic moment?
Vector quantity that represents magnetic strength and orientation of a magnetic source
- Tendency of an object to align with a magnetic field
- Arise from orbital motion of electrons and intrinsic spin of electrons
311
What direction is the orbital magnetic moment?
Perpendicular to the plane of the electrons orbit
312
How does the spin magnetic moment arise?
Arises from the intrinsic angular momentum of electrons
313
Which magnetic moment is the dominant factor?
The spin magnetic moment not the orbital magnetic moment
314
What are two types of interaction for magnetism?
Diamagnetism and paramagnetism
315
What is true about diamagnetism and paramagnetism?
Diagmagnetism usually have no unpaired electrons
- moment of two electrons cancel out so no net magnetic moment
Paramagnetism there is unpaired electrons there is permanent magnetic moment
316
Which magnetism effect is stronger?
Paramagnetism is stronger than diamagnetism
317
What is true about diamagnetic and paramagnetic materials?
In diamagnetic materials, on applying an external force, a magnetic moment is induced. The force of the interaction tends to push the atom out of the magnetic field
In paramagnetic materials, the unpaired electrons will align with the applied field, causing the atom or molecule to be attracted into the field
318
What can CFT be used to predict with magnetism?
Number of unpaired electrons in a complex and hence the type of magnetic interaction
319
What is the magnetic moment of a paramagnetic complex?
μeff
320
What are the two magnetic moments that make up μeff?
Spin angular momentum - spin contribution is the dominant and is determined by the number of unpaired electrons (n)
Orbital angular momentum - orbital contribution is a smaller effect (typically < 20%) and can be predicted but not quantified
321
How to calculate magnetic moment μso (spin only)?
(n(n+2))^1/2 * μB where μB is Bohr magnetons
1 μB = eh/4πm = 9.27x10^-24 JT^-1
where n is the number of unpaired electrons
322
What should you be able to predict from μso? And where do the small deviations of observed values from μso result from?
Should be able to predict number of unpaired electrons
Small deviations come from ignoring orbital angular momentum or spin-orbit coupling
323
What is the value of μso for a diamagnetic compound?
Equals zero or close to zero
324
How do you obtain μeff (effective magnetic moment)?
Can be obtained experimentally from the molar magnetic susceptibility (χm) using a Gouy balance
μeff = 2.83(χm*T)^1/2 where T is in K
325
What can be used instead of a Gouy balance to make it more accurate and how do you know if it is diamagnetic or paramagnetic?
A laser
- Diamagnetic if the laser points lower on calibration plate
- Paramagnetic if the laser points higher on calibration plate
326
What are the uses of magnetic measurements in the lab?
Can be used to distinguish between compound and their isomers
- Can be used with other techniques like NMR and colour
- Works due to differing amount of unpaired electrons in each isomer
327
What is the abundance of the lanthanides like?
Often termed 'rare earth elements' but are almost all relatively abundant on earth
328
What are the properties of the f-orbitals? (lanthanides)
Valence e- found in 4f orbitals (limited radial extension and therefore extremely contracted)
Leading to core-like orbitals so spatially unavailable for bonding
Largely unaffected by ligands attached to the metal
So no overlap with ligand orbitals and so f-orbitals do not particle in bonding
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What kind of interactions do f-block elements have?
Electrostatic interactions and not covalent bonding
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What oxidation state is most common in the lanthanides?
3+ dominates the chemistry of the lanthanides as the 4th ionisation energy is very large
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Why is the 4th ionisation energy of the lanthanides so large?
Once 3 electrons have been removed, stabilisation of 4f orbitals is so great the remaining 4f electrons are so tightly held as to be inaccessible
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When can the 2+ oxidation state be stable in the lanthanides?
Electronic stability is achieved at high filled (Eu2+), almost half filled (Sm2+) and completely filled (Yb2+) 4f shells
As a result very high values for the 3rd IR are observed for Eu and Yb and very low values for subsequent elements
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When can the 4+ oxidation state be stable in the lanthanides?
4th ionisation can be compensated by bond formation (strong bonding interactions with ligands help stabilise 4+)
- So 5 lanthanides display some tetravalent chemistry
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What is the lanthanide contraction?
The reduction of Ln metal and Ln3+ radii across the period
- Behaviour arises from the poor ability of the 4f electrons to screen the other valence electrons from the increasing nuclear charge
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What are the exceptions to the lanthanide contraction?
Metals Eu and Yb are exceptions - metallic radii about 0.2 A larger than expected
- Can be rationalised by viewing the electronic structure of these ions as being like 'cations in a sea of electrons'
Ln2+(2e-) vs Ln3+(3e-)
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What is true about the f-block and their similarity to other blocks when it comes to coordination chemistry?
Have as much in common with that shown by the s-block elements as it has with that of the d-block
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What are lanthanides and actinides and how does that affect their coordination preferences?
Hard lewis acids so coordinate preferentially to hard bases (F or H2O)
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What is the bonding in f-block complexes like?
Largely ionic in character due to lack of extension of the valence f orbitals meaning there is little interaction between metal orbitals and ligands
- Actinides show some covalency in their bonding due to 5f orbitals being more extended
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When looking at coordination chemistry, what is a consequence of the f-block elements being large?
They are large but typically high charge which gives them a high charge density
This large size and lack of a need for orbital overlap leads to high CN (often 9-12) determined by steric factors
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What is a common structure for f-block complexes and when can you get lower CNs?
Common structure is tricapped trigonal prismatic (prism with ligand in each corner and 3 on the faces)
Examples of lower CN exist with bulky ligands
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What happens to the formation constant K when denticity of a ligand increases?
Formation constant K increasing dramatically
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What is true about complexes of f-block elements that have monodentate ligands? What has this lead to?
They undergo extremely high rates of exchange
This kinetic instability has led to polydentate chelates being employed to form the ligand set (in particular macrocycles)
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How do we compare colour in lanthanides to transition metal complexes?
- Both f-f and d-d transitions are forbidden
- In lanthanides the radially contracted 4f orbitals don't interact strongly with ligands compared to the strong interaction for TMs
- Vibronic coupling is insignificant in Ln
- Less intense colours are observed in lanthanides
- There are sharp f-f absorption bands compared to broader d-d absorption bands
- Spectra are virtually independent of environment in lanthanides compared to a substantial influence of environment in TMs
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What is true about all elements beyond U in the actinides?
All elements beyond U have been synthesised in labs
- rely on bombardment of a suitable target with neutrons
- heavier elements require additional heavy ion bombardment
Far less developed than lanthanides due to reasons above and their radioactivity
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How can you compare 4f and 5f orbitals?
They have the same shape - same angular part but 5f has a radial node
5f has greater extension with respect to 6s and 6p than 4f with respect to 5s and 5p
- Therefore 5f is less shielded from influence of ligands
- Leading to greater covalent contribution to bonding
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What is nuclear fission?
Term given to the splitting of a large nucleus into two smaller ones
- Process also releases one or more neutrons which can collide with further nuclei
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What is required for a chain reaction of nuclear fission to occur?
A critical mass of fissile material
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What is generated from a nuclear fission chain reaction?
Thermal energy is harnessed and usually used to heat water to drive steam turbines and generate electricity
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What are radioisotopes?
Unstable isotopes of elements which undergo nuclear decay and emit some form of radiation
- Alpha
- Beta
- Gamma
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What is Alpha (ɑ) radiation?
Emission of an alpha particle (2 protons and 2 neutrons (Helium))
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What is beta (β) radiation?
Occurs when a neutron is transformed into an electron and a proton
Followed by emission of the electron from the nucleus into an electron cloud
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What is gamma (ɣ) radiation?
Emission of electromagnetic energy (gamma rays) from the nucleus (usually alongside one of the other types of decay)
- The identity of the elements remains unchanged
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What is the order of penetration of the types of radiation?
ɑ < β < ɣ
ɑ: stopped by a piece of paper or few cm of air
β: cut off by few mm thick Al sheet
ɣ: requires lead shielding to reduce intensity
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What is magnetism like in the lanthanides?
Magnetic properties of Ln3+ are a combination of spin and orbital angular momentum (spin-orbit coupling) - little influence from ligands
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What is magnetism like in the actinides compared to lanthanides?
Larger spatial extension of 5f vs 4f allows for much stronger interactions with neighbouring spin active species
- Led to suggestions that actinides combine good attributes from 3d and 4f metals
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Is there magnetism in the f-block?
There are many highly paramagnetic ions due to the presence of seven f-orbitals
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What blocks share similarities and differences with lanthanides and actinides?
s-block shows similarity with lanthanides (ionic bonding)
Lanthanide metals are quite dissimilar to d-block metals (covalency)
d-block share similarities with the actinides (covalency)
