Transition Metal Complexes

Transition metal complexes consist of a central Transition metal ion surrounded by a number of ligands.

As a result of their small size, these metal ions have a strong attraction for species which are rich in electrons.

LigandsLigands are negatively charged ions e.g.

Cl-, CN- or molecules containing one or more lone pairs of electrons e.g. NH3 which has one lone pair of electrons on the N atom and H2O which has two lone pairs of electrons on the O atom.

Ligands are electron donors i.e. they donate their non-bonding electrons into unfilled orbitals forming dative covalent bonds.

Ligands which donate one pair of electrons to the metal ion are said to be ‘monodentate’ e.g. Cl- and H2O.

Ligands which donate two pairs of electrons to the metal ion are said to be ‘bidentate’ e.g. the oxalate ion, C2O4

2-

EDTAEDTA (Ethylenediaminetetraacetic acid) is

classed as a hexadentate ligand as it donates six pairs of electrons to the metal ion.

It reacts with Ni2+ in a 1:1 ratio

The Co-ordination NumberThe number of bonds from the ligand to the central

metal ion is known as the co-ordination number of the metal ion e.g. when EDTA bonds to Ni2+, the Ni2+

ion has a co-ordination number of 6.

The co-ordination number of Cu2+ is 6 when it bonds to water molecules. This arrangement produces the blue colour of aqueous solution containing Cu2+

ions.

Shapes of complex ionsCentral metal ions with a co-ordination number

of 6 will be octahedral in shape.

Central metal ions with a co-ordination number of 4 tend to be tetrahedral in shape e.g. [CuCl4]2- and [CoCl4]2- but can be square planar e.g. Pt(NH3)2Cl2

In general, any given ion with a particular type of ligand will always tend to have the same shape.

The effect of ligands on degeneracyIn a Transition metal ion the five d-orbitals

within the 3d subshell are degenerate.

As ligands approach the metal ion along the x, y, and z axes, the electrons in the d-orbitals that lie along these axes (namely the dx

2- y

2 and the

dz2 orbitals) will be repelled by the electrons of

the approaching ligands.

As a result, these d-orbitals will now have a higher energy than those that lie between the axes (the dxy, dxz and dyz) and the d-orbitals are no longer degenerate.

The energy difference between the two sub-sets of d-orbitals depend on the position of the ligand in the ‘Spectrochemical Series’:

CN- > NH3 > H2O > OH- >F- > Cl- > Br- > I-

decreasing energy difference

‘Strong field’ ligands, e.g. CN-, cause a large energy difference between the two sub-sets of orbitals whilst ‘weak field’ ligands, e.g. I-, cause a small energy difference.