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Year 3 - Chemistry > Elec Spec of Metal Complexes > Flashcards

Elec Spec of Metal Complexes Flashcards

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1
Q

What is the Beer-Lambert Law?

A

A = log10(I0/I) = εcl

where I = intensity
I = intensity of incident light
c = concentration in M
l = path length of light in cm
ε = extinction coefficient (M-1cm-1)

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2
Q

When is the Beer-Lambert law valid?

A

Valid only at low absorbance (A < 1)

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3
Q

What is on the axis on an absorption spectra?

A

Absorbance (y-axis)
Wavenumber or wavelength (x-axis)

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4
Q

How can you convert between wavelength in nm to wavenumbers in cm-1?

A

λ (nm) = 107 / v (cm-1)

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5
Q

What is an eV in terms of kJ/mol and cm-1?

A

1 eV = 96.5 kJ/mol

1 eV = 8067 cm-1

where kT = 200 cm-1 @ 298 K

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6
Q

What is the range of visible light in λ and v?

A

λ: 380 - 750 nm

v = 26,500 - 13,000 cm-1

where the first number is violet and latter is red

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7
Q

What types of elec transitions occur in metal complexes?

A

d-d transitions

Ligand-to-metal charge transfers (LMCT)

Metal-to-ligand charge transfer (MLCT)

Ligand based

f-f and f-d transitions

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8
Q

When do d-d transitions occur?

A
  • Open-shell dn compounds
  • Depends on ligand-field splittings
  • Depends on electron repulsion
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9
Q

When do ligand-to-metal charge transfers (LMCTs) occur?

A

Occurs in all complexes, including d10

Reduces metal

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10
Q

When does metal-to-ligand charge transfer (MLCT) occur?

A

Requires ligands with low-lying π* orbitals
Such as CO, CN-, NO, bipy and phen

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11
Q

When do ligand-based transitions occur?

A

n-π* and π-π* transitions in delocalised systems

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12
Q

When do f-f and f-d transitions occur?

A

In lanthanides and actinides

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13
Q

What is the selection rule for an electric dipole transition to be observable?

A

∫ ψfinalμψinitial dτ =! 0

Where μ is TDM operator

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14
Q

How can you find the ligand-field ground state of a complex?

A

Use electron config
Find max S, then find degen in that state
Then letter is the degen, S is superscript
1/2 is symmetry of ψorbital under C4, if anti then is 2
g/u is symm from inversion

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15
Q

How can you find the number of microstates of a state?

A

Use letter to give the degen of states
Then spin multiplies them

2T suggests 6 (2x from spin and 3 states from T)

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16
Q

What is the ligand field state term for eg1?

A

s=1/2, S = 2
doubly degen so E

2Eg

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17
Q

When is a shoulder observed in electronic spectra?
(e.g. t2g1 to eg1)

A

Jahn-Teller distortion of the excited states

This is because some bonds longer than other (eg split further)

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18
Q

What are the term states of Ti3+ (d1) in Oh and Td environments?

A
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19
Q

What are the term states for hs d6 (such as Fe2+)?

A

Must look how to pair down spin e-
e- has been added to A1

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20
Q

What are the term states of hs d4 (such as Cr2+)?

A

Use A1 x “hole” in d5

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21
Q

What are the term states of d9 (such as Cu2+)?

A

Need to find orbital degen for arranging a hole

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22
Q

How many bands are observed in d-d bands in Oh/Td?

A

Oh/Td have one d-d band each

Oh: T2 -> E
Td: E -> T2

Any asymm band due to JT dist

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23
Q

What are the relative intensities of d-d bands?

A

Moderate intensities

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24
Q

What is the assumption of the weak-field approach?

A

Ligand field is a small perturbation to Russel-Saunders terms of the free ion

Is in the gas-phase

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25
Q

What is the weak-field state of d1?

A

5x microstates with different ML values
5xequal energy 3d states
Ground state is 2D

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26
Q

What is the weak-field state of d6?

A

e- added to spherical d5

Causes s=2, S=5, L=2
5D

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27
Q

What is the weak-field state of d4?

A

s=2, S=5, L=2

5D

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28
Q

What is the weak-field state of d9?

A

4xe- added to d5 sphere
s=1/2, S = 2, L=2
2D

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29
Q

What is the symmetry of free ions?

A

Spherical symm - R3

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30
Q

How can you find how a free ion term splits into fields in different fields?

A

Use descent in symm tables (O includes cubic ligand fields such as Oh and Td)

Look at term under R3 and find the orbitals which they split into

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31
Q

What is an orgel diagram?

A

Plots splitting of free ion terms and energy of microstates with increasing ligand field splitting (Δ)

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32
Q

How does the electric TDM operator transform as in Oh and Td symm?

A

x,y,z on RHS

In Oh: T1u
In Td: T2

33
Q

What is required for an allowed transition wrt symmetry?

A

Non-zero TDM means integrand must be an even function

Γ(ψfinal) x Γ(μ) x Γ(ψinitial) must contain A1

34
Q

What is the spin selection rule and when is it not followed?

A

ΔS = 0

Breaks down when ψspinψorbital cannot be separated due to spin-orbit coupling
This breaks as atoms get heavier

35
Q

What is the Laporte selection rule and when is it applicable?

A

ΔL = +/- 1 for Q =! 0

When spin and orbital wavefn can be separated

When centrosymm has parity rule of: g <-> u

36
Q

What does Laporte rule prevent?

A

Forbids: p-p / d-d / f-f transitions in atoms and ions

37
Q

Why do Oh have less intense transitions than Td?

A

Td is non-centrosymmetroic so doesn’t need to fufill parity part of Laporte rule -> less forbidden

Oh has to follow those rules

38
Q

What relaxes the parity part of Laporte rule?

A

Relaxed by vibronic coupling

Co-excite a vib mode of symm

39
Q

How does covalency effect the amount of vibronic coupling?

A

Covalency increases vib coupling

40
Q

How does vib coupling relax parity selec rule?

A

Separate ψ = ψelecψvib</sup

41
Q

What dictates band widths of transitions?

A

Co-excitation of vib modes

Progressions depends on Franck-Condon principle
Sharper - no change in r(M-L)
Broader - large change in r(M-L), more vib co-excitation

42
Q

Why do hs d5 complexes have broad and sharp bands?

A

Sharp - due to spin flips within the t2g and eg, no change in bonding

Broad due to covalency - large change in bond length from t2g -> eg
So large co-excitation of vib states

43
Q

What mixing occurs in Td complexes?

A

d/p mixing:
t2 from metal nd and (n+1)p orbitals
e is purely d

e -> t2 has some g -> u character, means the intensity is increased

44
Q

What is the intensity of π->π* transitions?

A

v large

as large dipole moment change

45
Q

How many transitions would you usually predict for d2,3,7,8 ions?

A

Would expect 3 transitions

(you dont)

46
Q

What are the free ion terms of d2?

A

s=1, S= 3

Max ML = 3, which implies 3F

Also see a 3P term

47
Q

How do d2 free ion terms split in ligand fields?

A

Within Oh/Td
3F -> 3A2 + 3T2 + 3T1

3P -> 3T1

Means there are 3x spin-allowed, laporte forbidden bands observable due to vib coupling mech

48
Q

What is the ligand ground state of Oh d2?

A

s=1, S=3, L=3 so T
9 possible microstates

t2g x t2g = A1g + Eg + [T1g] + T2g

Spin triplet associated with 1T1g

g.s is 3T2g

other states are singlet states

49
Q

What is the config interaction of d2?

A

Splitting of F term increases with Δ

Set difference in F and P states is 15B, which is the Racah parameter

50
Q

What is observed in the elec spectrum of d2?

A

In this case large Δ which causes A2g to raise above T1g

No band gives Δ

51
Q

What is the summary for d2,3,7,8 Oh elec spec?

A
52
Q

How many bands are observed in different dn?

A

dn:

0,4,6,9 - gives 1 band
5 - no bands
2,3,7,8 - gives 3 bands

53
Q

What is the summary Orgel diagram for Oh/Td d2,3,7,8?

A
54
Q

When are Orgel diagrams used?

A

Determine # of spin-allowed bands and symm of states in transitions

Weak field, high spin only, not for d5 high-spin

No info for spin-forbidden transitions

Qualtitative

55
Q

What is a Tanabe-Sugano diagram?

A

Δ (in terms of B) on x-axis
Energy (in terms of B) on y-axis

Quantitative version of Orgel

56
Q

What units are energies given in Tanabe-Sugano diagram?

A

In terms of B - the racah parameter

Where the racah term is from e- repulsion

All are relative to ground state

57
Q

How do Racah parameters B and C relate?

A

Both measures of e- repulsion

C/B ~ 4

58
Q

How can you find Racah parameters using Tanabe-Sugano diagrams?

A

Find 2 transition wavenumbers and their ration

Use difference in diagram (which is in terms of B) to git to best ratio

Then find B, Δ, and any other transition wavenumbers

59
Q

What suggests mixing occurs in a Tanabe-Sugano diagram?

A

Close in energy, around Δ/B = 10

This can give rise to spin-forbidden bands

60
Q

What is the nephelauxetic effect?

A

Reduced e- repulsion (and racah parameter) in complexes than free ions
Suggests increased average distance between e-

61
Q

What is the source of nephelauxetic effect?

A

Covalence of M-L bonding delocalises e-

So more spread out and lower B

62
Q

What is the nephelauxetic effect series?

A

Extent differnet ligands reduce B:

2nd/3rd row deecrease B most

F- < H2O < NH3 < en < ox < SCN- < Cl- < CN- < Br- < I-

63
Q

What occurs in a LMCT?

A

Ligand-to-metal charge transfer

Excitation of an e- frommainly ligand-based orbital to mainly metal-based orbital

64
Q

What are the symmetries in tetrahedral complexes which allow LMCT?

A

t2 MOs are σ and π in symm and so have contributions of both d and p orbitals

Ligand e- fill up to t1

65
Q

What are the LMCT transitions in Td complexes?

A

LMCT of increasing energy:

t1(π) -> e(π*)

t1(π) -> t2(σ*,π*)

t2(σ,π) -> e(π*)

t2(σ,π) -> t2(σ*,π*)

66
Q

What are the SALCs in Oh complexes?

A

σ: a1g + eg + t1u

π: t1g + t2g + t1u + t2u

67
Q

What are the LMCT excitations in Oh complexes?

A

LMCT transitions:

t1u(σπ) & t2u(π) -> t2g + eg

u x u x g -> g, allowed BUT
t1g(π) -> t2g(π*) is forbidden

68
Q

How does changing the ligand change frequency of LMCT transitions?

A

Less χ (electro-ve) ligands have less tightly held t1(π) e- at higher energy
Means v(LMCT) decreases down the group

69
Q

How does metal oxn state change freq of LMCT?

A

Increased oxn state lowers energy of d and e(π*) orbitals (where the orbitals promoted to)

Means v(LMCT) decreases from Fe(II) to Fe(III)

70
Q

How does freq of LMCT change across a series?

A

v(LMCT) decreases across a series as metal d orbitals stabilised by increasing nuclear charge

However at Zn(II) they have to promote to next orbital

71
Q

How does freq of LMCT change down the group?

A

v(LMCT) increase down group as metal d-orbital increases

This is due to 1/n2 factor in orbital energy

Can lead to higher elements being coloured and down the group being colourless (in UV)

72
Q

What bands are seen in [MnO4]-?

A

4xLMCT bands expected and observed

Mn-O lengthened in LMCT, co-excites A1 mode
This causes a progression with spacing the freq of A1 mode in excited state

73
Q

What does J-value size suggest about following Curie law?

A

Large J suggests follows the Curie law

As ligand-field effects negligible

74
Q

How does separation of states depend on J?

A

The separation is proportional to J

So larger values will have larger spacing

75
Q

Why does Eu(III) have a mag moment when not expected by spin-only?

A

1st es (excited state) v slightly higher in E than gs
This causes secondary zeeman effect and they gain angular momentum

Stat mech assumption not followed as gs not thermally isolated so doesnt follow Curie

76
Q

How does mag moment of Er(III) change at low T?

A

Only below 10K does diff between energy levels is close to kT
So accessible es

77
Q

What is the 2nd order Zeeman effect?

A

Seen in Eu(III)

Mixing es into gs, which is proportional to B2/energy separation

Not T dependent, doesnt effect Curie behaviour

78
Q

When does spin-orbit coupling or term symbols more important?

A

Term symbol - Lanthanides, with s-s coupling > l-l > s-l > LFE

SO coupling - 1st row TM, as s-s > l-l > LFE > s-l

79
Q

What does L values mean for Curie law?

A

Curie law when only one state available so L=0

For T dependent, L>0

Year 3 - Chemistry (12 decks)

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