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Year 2 - Chem > f-block > Flashcards

f-block Flashcards

1
Q

Why can free ion model be used for f-block elements?

A

Spectra is almost same in the gas, solid and liquid phase

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2
Q

How are f->f transitions present on an absorption spectra?

A

As the e- are not in bonds they are sharp transitions
Weak transitions as parity forbidden ( u->u)

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3
Q

What causes intense transitions for lanthanides in absorption spectra?

A

From allowed transitions:
u -> g
4f -> 5d

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4
Q

How does spin orbit coupling change across lanthanides?

A

Increases
Causes larger gaps at the same L

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5
Q

Why is lanthanide luminescence long-lived?

A

Forbidden nature of f-f transitions

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6
Q

What is the energy gap law?

A

Emissive state is state with largest energy gap to next state lower in E

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7
Q

What is the crystal effect like on lanthanides?

A

Crystal field lifts degen to give different mj states

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8
Q

What is the free ion approx?

A

4f orbitals don’t overlap sig with ligand orbitals
Ligand field splittings insig wrt kT

4f orbitals in Ln3+ are degen and I=3 for 4f

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9
Q

When does the spin-only formula apply to a lanthanide?

A

4f7 as equal occuption of all orbitals

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10
Q

How can the magnetic moment (μeff) be calculated?

A

μeff = gJx Sqrt[J(J+1)] x μb

In units of μb

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11
Q

How does the spin a.m. contribute to magnetic moment?

A

μeff = gs x Sqrt[s(s+1)] x μb

gs = 2
s = total spin q.n.

Only works for Gd but works for d-block

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12
Q

How does the half-life of actinides change across the series?

A

Half-life decreases across the series

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13
Q

How does the energy of f-orbitals change across the f-block?

A

Start: nf orbitals comparable to (n+2)s and (n+1)d

Across period then all drop in energy (drop is slower for actinides due to radial node in 5f)

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14
Q

How can lanthanides be described wrt ionic/covalent and why?

A

Ionic
Due to limited penetration of 4f
LFSE v. small

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15
Q

How can actinides be described wrt ionic/covalent and why?

A

More covalent than lactinides

Due to core e- in actinide contracting due to relativistic effect - mass so large than e- experience relativistic contraction

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16
Q

How do e- fill lanthanide orbitals?

A

General: 4fn6s2

Exceptions:
Ce, Gd, and Lu

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17
Q

Configuration fo electrons in Ce, Gd, and Lu?

A

Ce (low Zeff for 4f early in the series): 4f15d16s2

Gd (e- repulsion in 4f8): 4f75d16s2

Lu:
4f145d16s2

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18
Q

How does the 4f change across the period?

A

4f drops rapidly
Max in rdf moves inside 5p core sub-shell so core-like and no sig overlap with ligand orbitals

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19
Q

What are the results of charge-sensitivity of 4f Lanth electrons?

A

3+ ions have config 4fn

Bonding in 2+ or higher oxn states are highly ionic

Low oxn states are metallic but orbital overlap involves 5d and 6s orbtials (not 4f)

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20
Q

What is the trend of third ionisation energies?

A

Decrease from Sc->Y->La as n2 increases faster than Zeff2

General increase as Zeff2 increases faster and exchange energy increases to Eu

Dip at Gd & Lu as removing e- from higher energy 5d orbital

21
Q

What causes deviations from the ionisation energy trend?

A

1/4 & 3/4 shell effect

22
Q

What is the 1/4 & 3/4 shell effect?

A

Ionisation of 4f e- leads to decrease of L

Causes I3 to increase

1/4 includes: Pr, Nd, Pm
3/4 includes: Dy, Ho, Er

Effect larger in 3/4 as stronger e- repulsion

23
Q

How does atomisation energy change across the period?

A

General decrease from La -> Eu and Gd -> Yb

Metallic bonding requires promotion from 4f -> 5d/6s band, and 4f drops in E more than 5d/6s so more E required to form metallic bonds and atomisation energy decreases

Increase from Eu->Gd and Yb->Lu as Gd/Lu have 5d e- so stronger metallic bonding

2nd period higher than expected as reduces E of spin pairing, so promoting e- to regain this is easier

24
Q

How does the radius M3+ change across the lanthanides?

A

Radii decreases gradually due to poor shielding of 4f
Means Zeff increases gradually and LFSE small

Ions have higher charge density over period & attracts H2O more strongly so more energy released from Coulomb interaction

25
Q

What oxn states are present in lanthanides?

A

I3 compensated by high lattice & hydration enthalpies

I4 high so needs F- or O2- and either Ce, Tb, and Pr

I2 unstable wrt disproportionation other than Eu2+ & Yb2+, which has high I3

26
Q

How does the 5f change across the actinides?

A

Start: similar to 6d and 7s
Along the series 5f drops below but slower than 4f due to radial node in 5f

27
Q

What are the electron config of actinides?

A

5fn7s2 and 5fn-17s2 compete

5f -> 6d for more bonding e- much easier than 4f -> 5d for lanthanides

28
Q

How covalent are the actinides?

A

5f have greater radial extension than 4f so more covalent
So more complex forms, even with neutral/pi-bonding ligands

29
Q

What are the oxidation states of actinides?

A

Early actinides - high oxn formed easily
Radii contracts across the group

Up to +4 then can treat actinide ion as spherical
+5 and above then linear dioxo ions

30
Q

What is the structure of PrO2 and TbO2?

A

Fluorite structure

31
Q

What is the trend of disproportionation of +2 lanthanides?

A

Trend dominated by I3 and if large then stabilises

Includes:
Max -> Eu2+ (4f7) and Yb2+ (4f14)
Near max -> Sm2+ and Tm2+

32
Q

What is the coordination sphere of lanthanides?

A

Irregular as ionic bonding is non-directional and coordination geometry determined by ligand packing/repulsion

As ionic radii decreases across series there is a tendency for coordination number to drop from 8-9 (and 12 in complexes) to 6-8, especially when larger anions

33
Q

What structure do lanthanide halides have?

A

9 coordinate from La to Eu and 8 coordinate from Gd to Lu
Switch occurs earlier when heavier halide as more repulsion due to smaller cation

34
Q

How acidic are the aqueous lanthanides?

A

Hydrolysis increases across series as the ionic radius decreases

35
Q

What redox chemistry occurs in lanthanides?

A

Little redox chem except Ce4+/Ce3+ couple

36
Q

What is the stability of lanth complexes with neutal and anionic N/O ligands?

A

Neutral ligands - generally unstable, requires non-aq solution
Anionic ligands - form readily with hard acids

Thermo driving force is entropy gained when charges are neutralised on complex formation & water molecules solvation released from metal ion and anionic ligand

37
Q

How are mono/polydentate ligands favoured by lanthanides?

A

Unidentate - weakly coordinate, sometimes solvent coordinates in addition to poly
Polydentate - chelates more stable

38
Q

Do lanthanides form carbonyl complexes?

A

No as the 4f e- are not available for π-donation
Unlike carbonyl complexes

39
Q

How stable are organometallic lanthanide complexes?

A

Ionic bonding
Air and moisture sensitive -> hydrolysed instantly by water

Highly electrophillic as few anionic ligands to satisfy electroneutrality principle, so attract e- density from weakest donors (N2, alkenes)

40
Q

What are lanthanide complex with carbanionic ligands?

A

Not sterically hindered - dimeric/oligomeric/polymeric and insoluble in organic solvents

Sterically hindered - monomeric soluble compounds, and prevents ligand exchange

41
Q

Why are lanthanides good catalysts?

A

Strongly electrophilic catalysts so attract e- density from ligands
Weak interaction results in them activated for attack

42
Q

What actinides are stable in the 2+ oxn state?

A

Common for only heaviest actinides (Cf -> Lr)
Due to largest Zeff

Similar properties to Ba2+

43
Q

What actinides are stable in the +3 oxn state?

A

Most common, most stable after Am
Low IE for early one causes this
Similar properties to lanthanides

44
Q

What actinides are stable in the +4 oxn state?

A

Common for Th->Pu
Many dioxides (fluorite structure), oxy halides and tetrahalides
Th4+ similar chem to Zr4+ and Hf4+
V.oxidising after Am
Hydrolysed aquo ions, rich coordination and redox chem

45
Q

What actinides are stable in the +5 oxn state?

A

Found up to Am - only as F, oxy halides, oxides, and oxy anion salts
Aqueous: AnO2+ for U, Np, Pu, and Am
Principal oxidation state for Pa
Multiple bonding involves d and f

46
Q

What actinides are stable in the +6 oxn state?

A

Important oxn for uranium -> uranyl ion UO22+
Same ions also found for Np, Pu, Am

47
Q

What actinides are stable in the +7 oxn state?

A

Highly oxidising
NpO53- and PuOsub>5</sub>3-

48
Q

How is uranium isolated?

A

Uranium ore -> UO22+
1st step: ion exchange chromatography then tri-n-butylphosphate
Then:
UO22+ + heat -> UO3
UO3 + H2 + heat -> UO2
UO2 + HF + heat -> UF4
UF4 + Mg + heat -> U

Year 2 - Chem (23 decks)

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