CHM 501 Lecture


Crystal Field Theory or Ligand Field Theory

An ionic approach to understanding bonding in transition metal complexes

Consider the Oh case : what is the effect of the negative electron density from the ligands on the energies of the valence (d) orbitals

Δo = 10Dq = Crystal Field Splitting parameter or Ligand Field parameter

Occupation of the t2g orbitals gives a little extra stabilization of the complex

Ligand Field Stabilization Energy (LFSE)

d electron
configuration
Crystal Field
configuration
LFSE
number of
unpaired spins
d1
t2g1
–4Dq
1
d2
t2g2
–8Dq
2
d3
t2g3
–12Dq
3
d4
t2g4
–16Dq + P
2 (ls)
d4
t2g3eg1
–6Dq
4 (hs)
d5
t2g5
–20Dq + 2P
1 (ls)
d5
t2g3eg2
0
5 (hs)
d6
t2g6
–24Dq + 2P
0 (ls)
d6
t2g4eg2
–4Dq
4 (hs)
d7
t2g6eg1
–18Dq+ P
1 (ls)
d7
t2g5eg2
–8Dq
3 (hs)
d8
t2g6eg2
–12Dq
2
d9
t2g6eg3
–6Dq
1
d10
t2g6eg4
0
0

P = spin pairing energy

hs = high spin

ls = low spin

Influences on Dq:

metal : charge, size (Z*)

ligands : charge, orbitals available for bonding (σ, π), ring formation

Spectrochemical series

ligands ordered by relative size of Dq for any metal ion ligands ordered by ligand field strength

I < Br < S2– < SCN < Cl < NO3 < F < ox2– < H2O < SCN < CH3CN < NH3 < en < bipy < phen < NO2 < PPh3 < CN < CO


High spin vs low spin

Complexes are high spin if Dq is small : weak field case

Complexes are low spin if Dq is large : strong field case

Magnetic susceptibility measurements, which measure the size of the magnetic moment, are used to distinguish the spin state.

The magnetic moment reflects the total available angular momentum in transition metal complexes (especially first row), most of this comes from spin (the orbital contribution is said to be quenched):

μ = g[S(S+1)]½μB

g ~ 2 (fundamental constant); μB = Bohr magneton

Since each unpaired electron has S = ½

μ = [n(n+2)] ½ μB

n = number of unpaired spins