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CHEM214 > Ligand Field Theory > Flashcards

Ligand Field Theory Flashcards

1
Q

What is ligand field theory?

A

It considers the overlap of ligand and metal atomic/molecular orbitals

It is NOT an electrostatic model

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2
Q

How’re bonds formed?

A

Bonds are formed by the overlap of atomic orbitals to produce molecular orbitals

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3
Q

How many ∂-donor orbitals are there in an octahedral complex?

A

There are 6 ligand sigma-donor orbitals

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4
Q

What rule is always upheld in ligand field theory?

A

We must start and finish with the same number of orbitals

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5
Q

What does a1g mean?

A

“a” means there is only one hybrid orbital in the set

s - orbital

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6
Q

What does eg mean?

A

“e” means there’s are 2 hybrid orbitals in the set

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7
Q

What does t1u mean?

A

”t” means there are three hybrid orbitals in the set

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8
Q

What’re the rules for orbital overlap?

A

Ligand and metal orbital must have same symmetry label to form MO - this is an absolute requirement

Atomic orbitals should be of similar energy and in the same region of space - proximity - to have strongest interaction.

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9
Q

How many molecular orbitals are formed from t1u hybrid ligand orbitals?

A

6 total
- 3 bonding
- 3 anti bonding

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10
Q

What’re t1u hybrid orbitals analogous to?

A

They’re analogous to p-orbitals

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11
Q

What d-orbitals are involved in forming ligand hybrid orbitals?

A

dx^2-y^2 and dz^2

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12
Q

How many molecular orbitals are formed from eg ligand hybrid orbitals?

A

4 total
- 2 bonding
- 2 anti bonding

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13
Q

What d orbitals form non-bonding molecular orbitals? Why?

A

dxy, dxz and dyz

Because there is no ligand hybrid with the correct “shape” / symmetry label to overlap with these metal d-orbitals

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14
Q

What do strong overlaps result in?

A

Stronger overlaps cause a greater energy difference/split between the bonding and anti-bonding combination

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15
Q

What is the order of the MO energy level diagram of sigma-bond interactions?

A

Lowest energy
a1g - 1
t1u - 3
eg - 2
t2g - 3 (non-bonding)
eg* - 2 (non-bonding)
a1g* - 1
t1u* - 3
Highest Energy

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16
Q

How many electrons are donated by the ligands to form sigma bonds in an octahedral complex?

A

12 electrons total
- 6 bonding MOs formed

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17
Q

How does symmetry tell you about the ligand sigma-donor hybrid orbitals for any geometry?

A

The symmetry tells you which ligand donor hybrid orbitals will overlap with which metal valence orbitals to form MOs.

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18
Q

How many metal valence orbitals are there in a tetrahedral complex?

A

9
s, px, py, pz, 5 d-orbitals

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19
Q

What symmetry must the ligand donor pair hybrids have in a tetrahedral complex?

A

Ligand donor pair hybrids must have a a1 and t2 symmetry

TETRAHEDRAL

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20
Q

What is the symmetry label of 4s?

A

a1

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21
Q

What is the symmetry label of dx^2-y^2 and dz^2?

A

eg

22
Q

What is the symmetry label of dxy, dxz, dyz?

A

t2g

23
Q

What is the symmetry label of 4p?

A

t1u

24
Q

What ligand hybrid orbitals have the correct symmetry to form MOs in a tetrahedral complex?

A

a1 and t2
- doesn’t include metal 4p orbital

25
Q

Whats the issue with CFT?

A

Gives no information on MOs

Does not identify L->M bonding orbitals

Cannot treat covalent interactions or multiple bonds

26
Q

What’re the two ways CO and a metal can bond?

A

Sigma donation

Pi back donation

27
Q

What is sigma donation wrt CO ligands?

A

Donation of 2 electrons from full non-bonding orbital on C
- C-O bond is unaffected

28
Q

What is pi back donation wrt CO ligands?

A

Donation from full d-orbital on metal to pi anti-bonding CO orbital on ligand
- C-O bond is weakened

29
Q

Which atom has greater electron density in a CO molecule?

A

Carbon, this is the atom that donates 2 electrons when bonding to metal ions

30
Q

What is the symmetry label for pi bonding?

A

t2g

31
Q

Why can s orbitals not bond to p orbitals?

A

Because they have different symmetry labels - overall overlap is zero

They therefore have a non-bonding interaction

32
Q

How many symmetry based combinations are there in octahedral complexes?

A

12 total

33
Q

What’re the combination of symmetry labels in an octahedral complex?

A

t1g, t1u, t2g, t2u

34
Q

What metal orbitals can form MOs through pi bonding? Why?

A

Any in the t2g set. They have the same symmetry as the ligand t2g pi anti-bonding orbital, they are also close together and have similar energies

dxy, dxz and dyz

35
Q

Why are the interactions of the orbitals in t1u symmetry label ignored?

A

Because the energy difference is so large for the bonding/anti-bonding t1u MOs.

Results in very weak interaction and so is ignored

36
Q

What is the only significant pi-interaction we need?

A

The only pi-interaction we need is between the non-bonding t2g set of metal d-orbitals and t2g ligand hybrids

37
Q

What is the LUMO on a pi-acceptor ligand?

A

The pi anti-bonding orbital

38
Q

What effect do pi-acceptor ligands have ∆oct?

A

They increase ∆oct as they reduce the t2g

39
Q

How does pi-back-bonding work wrt CO?

A

The new t2g MOs formed in pi-bonding contain any metal d-electrons

These form a pi-back-bond to the ligand LUMO - pi-anti-bonding orbital which weakens the C-O bond

40
Q

What effect do pi-donor ligands have on ∆oct?

A

They reduce ∆oct

41
Q

What is required for pi-donor ligands to form bonds?

A

Metal t2g set must have appropriate space - can fill eg* as this is non-bonding

42
Q

Why do halides reduce ∆oct so much?

A

Because halides have full px and py orbitals - these then pi-donate to the t2g set, reducing ∆oct.

The new t2g* MOs contain any metal d-electrons.

They also have bigger orbitals and so have greater overlap of p-orbitals - reducing ∆oct even more

43
Q

What is ∆oct between in pi-donor interactions? How does this differ to pi-acceptor interactions?

A

∆oct is from t2g* to eg*

This is smaller than t2g to eg* which is seen in pi-acceptor interactions

44
Q

What effect does a pi-acceptor ligand have on the energy of the d orbitals in the t2g set?

A

Pi-acceptor ligands reduce the energy of the d orbitals in the t2g set

M->L pi back-donation

(dxy, dxz, dyz)

45
Q

What effect does a pi-donor ligand have on the energy of the d orbitals in the t2g set?

A

Pi-donor ligands increase the energy of the metal d orbitals in the t2g set - reducing ∆oct

L->M donation

(dxy, dxz, dyz)

46
Q

What happens to the IR stretch of C-O bond in CO in the presence of MORE pi-back bonding?

A

MORE pi-back bonding means shorter M-C bond, longer C-O bond

Hence C-O IR stretch is of lower energy / wavenumber

47
Q

Are alkyl phosphines good/bad donors/acceptors?

A

Alkyl phosphines are:
Strong donors
Poor acceptors

Alkyls groups are e- donors

48
Q

What’s t he importance of pi-acceptor ligands?

A

Transfer of electron density from M->L stabilises metal centres in low oxidation states

Important applications in low oxidation state metal complexes

49
Q

How does electron density at a metal centre affect the IR cm-1 of CO?

A

Increased electron density at metal centre (lower oxidation state) causes cm-1 to fall - more pi-back bonding

50
Q

Are phosphites/phosphorus halides good/bad donors/acceptors?

A

Phosphites and phosphorus halides are:
Poor donors
Strong acceptors

Electronic effects - EWGs

51
Q

What are the IR wavenumbers of CO ligands?

A

Terminal is highest Vco = 1850-2300cm-1
- typical of CO

Bridging CO stretch is less than terminal Vco = 1750-1850cm-1
- typical of C=O

Face capping is lowest Vco = <1750cm-1

52
Q

What has a higher 13C NMR ppm (more deshielded), terminal or bridging CO?

A

Bridging and face capping are higher - 220-260ppm

Terminal is less - 190-210ppm

CHEM214 (7 decks)

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