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Coordination Chemistry
Bonding in transition-metal complexes
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
Ligand/d orbital interactions
Orbitals point at ligands
(maximum repulsion)
Orbitals point
between ligands
(less pronounced repulsion)
The two effects of the crystal field
Splitting of d orbitals in an octahedral field
eg
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
The magnitude of the splitting
(ligand effect)
Strong
field
Weak
field
The spectrochemical series
CO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I-
The magnitude of the splitting
(metal ion effect)
Strong
field
Weak
field
D increases with increasing formal charge on the metal ion
D increases on going down the periodic table
Placing electrons in d orbitals
Strong field
Weak field
Strong field
Weak field
d1
d2
d3
d4
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
Pairing Energy, P
The pairing energy, P, is made up of two parts.
1) Coulombic repulsion energy caused by having two electrons in
same orbital. Destabilizing energy contribution of Pc for each
doubly occupied orbital.
2) 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
Placing electrons in d orbitals
d5
1 u.e.
5 u.e.
d6
0 u.e.
4 u.e.
d8
2 u.e.
2 u.e.
d7
1 u.e.
3 u.e.
d9
1 u.e.
1 u.e.
d10
0 u.e.
0 u.e.
Positive
favors high
spin. Neg
favors low
spin.
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
 accepting ligands increase splitting and may be low spin
D: CO, CN-, > phenanthroline > NO2- > NCS-
Splitting of d orbitals in a tetrahedral field
t2
Dt
e
Dt = 4/9Do
Always weak field (high spin)
A crystal-field aproach: from octahedral to tetrahedral
L
L
L
M
L
L
M
L
L
L
L
L
Less repulsions along the axes
where ligands are missing
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
s  n(n  2)
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.