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Chemistry Of The Elements > Magnetic and spectral properties of TM properties > Flashcards

Magnetic and spectral properties of TM properties Flashcards

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1
Q

Singlet

A

spins of electrons are in different directions

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2
Q

Triplet

A

spins of electrons are in the same direction

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3
Q

Selection rules for electron transitions

A

the selection rules tell us about the probability of a transition and hence the intensity of the resulting band in the spectrum

  • spin selection rule: ΔS = 0
  • laporte selection rule: a change in parity is required, since a photon has angular momentum
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4
Q

Approximate wavelength ranges of the different colours (nm)

A

red: 700-620
orange: 620-580
yellow: 580-560
green: 560-490
blue: 490-430
violet: 430-380

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5
Q

Why are electronic spectra bands broadened?

A

spectra are broadened through the effects of molecular vibrations and rotations

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6
Q

Molar extinction coefficient, ε

A

the proportionality constant: how intrinsically intense the absorption is (can be defined at any wavelength)

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7
Q

What do ⫪-donor ligands do to Δ o ?

A

small Δ o

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8
Q

What do ⫪-acceptor ligands do to Δ o ?

A

large Δ o

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9
Q

What is the energy of transitions for d 1 , d 4 (high-spin), d 6 (high-spin), and d 9 ?

A

the energy of the transition is equal to Δ o

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10
Q

What are the energy of transitions for d 2 , d 3 , d 7 (high spin) and d 8 ?

A

complexes of these ions show more than one band - usually two or three - of very different energies

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11
Q

What do electron-electron repulsions depend on?

A

they depend on the arrangement of the electrons in the different d orbitals and on their spin
(repulsion is different according to which d orbitals the electrons occupy).

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12
Q

Russell-Saunders coupling scheme

A

spin-spin: coupling of spin angular momenta of electrons
→ S then 2S+1

orbit-orbit: coupling of orbital angular momenta
→ L

spin-orbit: coupling of the spin and orbital angular momenta
→ J
(only a small effect for 1st row TM)

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13
Q

Term symbol

A

a kind of ‘shorthand’ describing the energy, angular momentum and spin multiplicity of an atom in any state

(Different terms have different energies)

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14
Q

How to identify lowest-energy states using Hund’s Rules

A
  1. The ground state has the largest spin multiplicity (2S+1)
  2. If two terms have the same spin multiplicity, the term with the larger value of L is the lowest in energy
  3. For <1/2 filled subshell, ground state has lowest J and vice versa
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15
Q

What is the “weak-field approach”?

A

it assumes that the effect of the ligand field splitting is substantially less than that of interelectronic repulsion

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16
Q

What is the “strong-field approach”?

A

where the ligand field splitting is assumed to be much larger than the interelectronic repulsion

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17
Q

What do orgel diagrams show?

A

the effect of ligand field on the relative energies of the terms

(energy v splitting plots)

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18
Q

Requirements for orgel diagrams

A
  • High-spin cases only
  • Spin-allowed transitions only (i.e. only those states having the same spin multiplicity as the ground state are included - except for d 5 as none are spin-allowed)
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19
Q

What is an electron hole?

A

a single electron in an empty d-orbital
(same as a +ve charge in a filled d-shell)

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20
Q

What are term symbols used for in spectroscopy?

A

To describe the electronic states of atoms or ions based on total spin (S), orbital angular momentum (L), and total angular momentum (J).

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21
Q

What is the spin multiplicity of a singlet state?

A

1 (i.e. 2S+1=1, so S=0)

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22
Q

How do you determine the ground term symbol?

A

Use Hund’s rules:
- Maximum spin
- Then maximum L
- For < 1/2 filled subshell, lowest J
- For >1/2 filled subshell, highest J

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23
Q

What do Orgel diagrams represent?

A

Orgel diagrams show the energy levels of high-spin d-electron configurations in weak field ligands

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24
Q

For which d-electron configurations are Orgel diagrams useful?

A

Mostly for d 1 to d 7 in high-spin complexes since d 8 and d 9 are too complex

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25
What is the difference between Orgel and Tanabe-Sugano diagrams?
Orgel diagrams are qualitative and only for high-spin. Tanabe-Sugano diagrams handle both high- and low- spin and give quantitative splitting.
26
What determines the relative energy levels in an Orgel diagram?
Electron–electron repulsion and the crystal field splitting from ligands
27
What transitions are shown in Orgel diagrams?
Only spin-allowed transitions
28
What do Tanabe-Sugano diagrams show?
they plot the energy levels of electronic states for a specific dn configuration as a function of ligand field strength (Δo)
29
What is B and what does it measure?
B = the "Racah" parameter It measures repulsion of terms of the same multiplicity
30
How does the Racah parameter vary from free ions to complexes?
B is often reduced in complexes from the free ion value due to covalency (e.g. pairing energies are lower in complexes)
31
What is the Nephelauxetic effect?
the reduction in interelectronic repulsion between d-electrons of a transition metal when it forms a coordination complex
32
Why does the Nephelauxetic effect occur?
Ligands allow the d-electrons to delocalise more, which spreads out their charge, reducing repulsion
33
When are bandwidths likely to be broad?
when the gradient of the ground state and excited states on Orgel/ Tanabe-Sugano diagram are very different from one another as it means the magnitude of E is very sensitive to small changes in Δ and therefore to vibrations
34
Why can transitions occur in octahedral complexes when electronically forbidden?
if the transition is vibronically allowed
35
Why are there stronger absorption of tetrahedral vs octahedral?
p-d mixing in tetrahedral complexes relaxes the Laporte selection rule (p and d orbitals have the same symmetry)
36
How is the spin selection rule relaxed?
By spin-orbit coupling (a spin-forbidden transition may be observed if it has similar energy to a spin-allowed transition)
37
Why do first-row transition metal d-d states not emit light?
They absorb light instead because an electronic excited state energy may be converted into vibrational energy on a timescale of 1 picosecond (10 -12 s) which is much faster than for the emission of a photon.
38
Why do charge-transfer transitions tend to have higher intensity absorptions?
the Laporte selection rule doesn't apply
39
How can LMCT be interpreted?
- Flow of charge - Oxidation of the ligand, reduction of the metal - Process occurs within the timescale of absorption of a photon by the complex (~10 -16 s) - Rapid charge recombination (once the charge is transferred, there is a large thermodynamic driving force for the charge to return to its groundstate - these states don't last long)
40
What metals favour LMCT?
metals in high oxidation states
41
LMCT in tetrahedral d 0 complexes
MnO 4 - intense purple Cr 2 O 4 2- intense yellow Cr 2 O 7 2- (acidified chromate solution) intense orange
42
What metals favour MLCT?
Metals in low oxidation state (relatively easy to oxidise)
43
What is the energy of MLCT related to?
- the ease of oxidation of the metal - the ease of reduction of the ligand
44
Requirements of MLCT
- Likely to occur at low energy - Requires metal to be in low oxidation state - Requires ligands to have low-lying acceptor orbitals, usually ⫪*
45
What technique gives direct experimental evidence of MLCT?
Time-resolved infrared spectroscopy (TRIR)
46
How does time-resolved infrared spectroscopy (TRIR) work?
IR absorption spectroscopy of a molecule while in its electronically excited state. The IR stretches of reporter groups, such as the C=O group of esters, shift to lower frequencies in the excited state, indicating a weakening of the C=O bond due to the partial population of the ⫪* orbital.
47
What are the main classes of bulk magnetism?
1. Diamagnetism 2. Paramagnetism 3. Ferromagnetism 4. Ferrimagnetism
48
Diamagnetism
- Exhibited in substances with paired electrons - Application of external magnetic field causes a 'distortion' of the electronic orbital currents (i.e. the sample is repelled against the applied field)
49
Paramagnetism
- Exhibited in substances with unpaired electrons - It is caused by the circulation of unpaired electrons (generates a magnetic field directed with the applied field) -Expressed as the magnetic moment (𝜇)
50
When can orbital contribution to 𝜇 occur?
If it is possible to transform the orbital occupied by an unpaired electron into an equivalent orbital by rotation around the direction of the applied field. This means spin-only formula cannot be used.
51
When is orbital contribution expected?
if the ground-state term has T symmetry but NOT if E or A symmetry
52
Why doesn't spin-only formula fully work for A and E ground terms?
due to spin-orbit coupling (SOC) - this only works with orbital contributions from higher T terms of the same spin multiplicity as the ground state (if there are any)
53
𝜇 eff of complexes with T ground terms
- Temperature has a large effect on the magnitude of 𝜇 eff - 𝜇 eff is almost always larger than 𝜇 spin-only due to an orbital contribution to the magnetism
54
Kotani Plots
Used to show the effect of spin-orbit coupling on the orbitally degenerate ground term
55
What does the Curie Law imply?
- Each atom magnetic moment acts independently at all temperatures down to 0K - Interactions with other magnetic moments don't occur - Plotting 1/𝛘 against T will give a straight line passing through the origin at 0K
56
Ferromagnetic materials
ALL SPINS ALIGNED - the magnetic moments are adjacent atoms align parallel to each other spontaneously, without an external magnetic field
57
What is the Curie Temperature, T c ?
For ferromagnets, 1/𝛘 becomes zero at a temperature above 0K
58
Antiferromagnetic materials
SPINS CANCEL OUT EACH OTHER - Adjacent magnetic moments align antiparallel (opposite directions) and their magnitudes are equal - Occurs below the Néel temperature (T N - Above the Néel temperature, normal paramagnetism occurs
59
Factors that can cause spin crossover
- Change in temperature - Change in pressure - Irradiation with light
60
What spin state do high temperatures favour?
High temperatures favour the high-spin form as the greater amplitude of thermal M-L vibrations leads to a reduction in ligand field strength Δ
61
Characteristics of ferromagnetic materials
- They exhibit strong, permanent magnetism - High magnetic susceptibility - Becomes paramagnetic above the Curie temperature
62
Ferrimagnetic materials
the spins are also antiparallel but their magnitudes are unequal (therefore there is still a magnetic moment)

Chemistry Of The Elements (2 decks)

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