Ligand Field Theory Flashcards
What is ligand field theory?
It considers the overlap of ligand and metal atomic/molecular orbitals
It is NOT an electrostatic model
How’re bonds formed?
Bonds are formed by the overlap of atomic orbitals to produce molecular orbitals
How many ∂-donor orbitals are there in an octahedral complex?
There are 6 ligand sigma-donor orbitals
What rule is always upheld in ligand field theory?
We must start and finish with the same number of orbitals
What does a1g mean?
“a” means there is only one hybrid orbital in the set
s - orbital
What does eg mean?
“e” means there’s are 2 hybrid orbitals in the set
What does t1u mean?
”t” means there are three hybrid orbitals in the set
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.
How many molecular orbitals are formed from t1u hybrid ligand orbitals?
6 total
- 3 bonding
- 3 anti bonding
What’re t1u hybrid orbitals analogous to?
They’re analogous to p-orbitals
What d-orbitals are involved in forming ligand hybrid orbitals?
dx^2-y^2 and dz^2
How many molecular orbitals are formed from eg ligand hybrid orbitals?
4 total
- 2 bonding
- 2 anti bonding
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
What do strong overlaps result in?
Stronger overlaps cause a greater energy difference/split between the bonding and anti-bonding combination
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
How many electrons are donated by the ligands to form sigma bonds in an octahedral complex?
12 electrons total
- 6 bonding MOs formed
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.
How many metal valence orbitals are there in a tetrahedral complex?
9
s, px, py, pz, 5 d-orbitals
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
What is the symmetry label of 4s?
a1
What is the symmetry label of dx^2-y^2 and dz^2?
eg
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
