Ligand Field Theory Flashcards by Barney Burden

What is ligand field theory?

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

It is NOT an electrostatic model

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How’re bonds formed?

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

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

There are 6 ligand sigma-donor orbitals

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What rule is always upheld in ligand field theory?

We must start and finish with the same number of orbitals

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What does a1g mean?

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

s - orbital

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What does eg mean?

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

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What does t1u mean?

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

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What’re the rules for orbital overlap?

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

6 total
- 3 bonding
- 3 anti bonding

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What’re t1u hybrid orbitals analogous to?

They’re analogous to p-orbitals

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

dx^2-y^2 and dz^2

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

4 total
- 2 bonding
- 2 anti bonding

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

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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What do strong overlaps result in?

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

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

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

12 electrons total
- 6 bonding MOs formed

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

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

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

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

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

Ligand donor pair hybrids must have a a1 and t2 symmetry

TETRAHEDRAL

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What is the symmetry label of 4s?

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

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

t2g

What is the symmetry label of 4p?

t1u

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

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

Whats the issue with CFT?

Gives no information on MOs Does not identify L->M bonding orbitals Cannot treat covalent interactions or multiple bonds

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

Sigma donation Pi back donation

What is sigma donation wrt CO ligands?

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

What is pi back donation wrt CO ligands?

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

Which atom has greater electron density in a CO molecule?

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

What is the symmetry label for pi bonding?

t2g

Why can s orbitals not bond to p orbitals?

Because they have different symmetry labels - overall overlap is zero They therefore have a non-bonding interaction

How many symmetry based combinations are there in octahedral complexes?

12 total

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

t1g, t1u, t2g, t2u

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

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

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

Because the energy difference is so large for the bonding/anti-bonding t1u MOs. Results in very weak interaction and so is ignored

What is the only significant pi-interaction we need?

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

What is the LUMO on a pi-acceptor ligand?

The pi anti-bonding orbital

What effect do pi-acceptor ligands have ∆oct?

They increase ∆oct as they reduce the t2g

How does pi-back-bonding work wrt CO?

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

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

They reduce ∆oct

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

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

Why do halides reduce ∆oct so much?

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

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

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

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

Pi-acceptor ligands reduce the energy of the d orbitals in the t2g set M->L pi back-donation (dxy, dxz, dyz)

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

Pi-donor ligands increase the energy of the metal d orbitals in the t2g set - reducing ∆oct L->M donation (dxy, dxz, dyz)

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

MORE pi-back bonding means shorter M-C bond, longer C-O bond Hence C-O IR stretch is of lower energy / wavenumber

Are alkyl phosphines good/bad donors/acceptors?

Alkyl phosphines are: Strong donors Poor acceptors Alkyls groups are e- donors

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

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

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

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

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

Phosphites and phosphorus halides are: Poor donors Strong acceptors Electronic effects - EWGs

What are the IR wavenumbers of CO ligands?

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

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

Bridging and face capping are higher - 220-260ppm Terminal is less - 190-210ppm