1. Coordination Chemistry Bonding in transition-metal complexes

2. with the negatively charged ligands
Crystal field theory: an electrostatic model
The metal ion will be positive and therefore attract the negatively charged ligands
But there are electrons in the metal orbitals, which will experience repulsions
with the negatively charged ligands

3. Ligand/d orbital interactions
Orbitals point at ligands
(maximum repulsion)
Orbitals point
between ligands
(less pronounced repulsion)

4. The two effects of the crystal field

5. Splitting of d orbitals in an octahedral fieldeg
3/5 Do Do
2/5 Do
t2g
Do is the crystal field splitting
E(t2g) = -0.4Do x 3 = -1.2Do
E(eg) = +0.6Do x 2 = +1.2Do

6. The magnitude of the splitting (ligand effect)
Weak
field
Strong
field
The spectrochemical series
CO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I-

7. The magnitude of the splitting (metal ion effect)
Weak
field
Strong
field
increases with increasing formal charge on the metal ion
increases on going down the periodic table

8. Placing electrons in d orbitals
Strong field Weak field
Strong field Weak field d1 d2 d3 d4

9. When the 4th electron is assigned it will either go into the higher energy eg orbital at an energy cost of D0 or be paired at an energy cost of P, the pairing energy. d4
Strong field =
Low spin
(2 unpaired)
Weak field =
High spin
(4 unpaired)
P < Do
P > Do
Notes: the pairing energy, P, is made up of two parts. 1) Coulombic repulsion energy caused by having two electrons in same orbital

10. P = sum of all Pc and Pe interactions
Pairing Energy, P
The pairing energy, P, is made up of two parts.
Coulombic repulsion energy caused by having two electrons in same orbital. Destabilizing energy contribution of Pc for each doubly occupied orbital.
Exchange stabilizing energy for each pair of electrons having the same spin and same energy. Stabilizing contribution of Pe for each pair having same spin and same energy
P = sum of all Pc and Pe interactions

11. Placing electrons in d orbitals
1 u.e.
5 u.e. d5
0 u.e.
4 u.e. d6
1 u.e.
3 u.e. d7
2 u.e. d8
1 u.e. d9
0 u.e.
d10

12. Positive favors high spin. Neg favors low spin.

13. Spectrochemical Series
Purely s ligands:
D: en > NH3 (order of proton basicity)
donating which decreases splitting and causes high spin:
D: H2O > F > RCO2 > OH > Cl > Br > I (also proton basicity)
Adding in water, hydroxide and carboxylate
D: H2O > F > RCO2 > OH > Cl > Br > I
p accepting ligands increase splitting and may be low spin
D: CO, CN-, > phenanthroline > NO2- > NCS-

14. Splitting of d orbitals in a tetrahedral fieldDt e
Dt = 4/9Do
Always weak field (high spin)

15. A crystal-field aproach: from octahedral to tetrahedral
Less repulsions along the axes
where ligands are missing

17. Magnetic properties of metal complexes
Diamagnetic complexes
very small repulsive interaction with external magnetic field
no unpaired electrons
Paramagnetic complexes
attractive interaction with external magnetic field
some unpaired electrons

18. Measured magnetic moments include contributions from both spin and orbital spin. In the first transition series complexes the orbital contribution is small and usually ignored.