The thesis is concerned with the synthesis, characterisation and reactivity of a variety of nickel(II) diphosphine complexes considered possible catalyst precursors for the copolymerisation of CO and ethene. [Ni(diphos)(o-tolyl)(py)]Y (1), [Ni(diphos)(_2)](Y)(_2) (2), and [Ni(diphos)(S)(_2)](Y)(_2) (3) {Y = OTs, OTf} are found to be catalyst precursors for the CO/ethene reaction (115 C, 1200psi, 1:1 CO: ethene) with high selectivity to polyketone formation, the activity depending on the phosphine ligand employed. However, the activity is low and attributed in part to the formation of the [Ni(diphos)(CO)(_2)] (4) complexes which are stable under the reaction conditions. The diphosphine hgand employed, particularly the nature of the backbone, is found to influence the synthesis and reaction chemistry of the complexes (1), (2) and (3).A series of complexes of the type [Ni(R(_2)P(CH(_2))(_a)PR(_2))(o-tolyl)Br] (5) {n = 2; R = Ph, Cy, 'Bu) have been synthesised and the complex [Ni(dppe)(o-tolyl)Cl] has been characterised by X-ray crystaliography. The influence of the chelating diphosphine on the structural characteristics and NMR parameters of these compounds has been investigated. When n > 2 the complexes of the type [Ni(diphos)(o-tolyl)X] were not isolated. The reaction of type (5) complexes with TIY (Y = OTs, BF(_4)) in pyridine led to the formation of complexes of the type [Ni(diphos)(o-tolyl)(py)]Y (1).A series of complexes of the type [Ni(diphos)Cl(_2)] (6) {diphos = dope, dBpe, dcpn} have been synthesised and characterised by X-ray crystallography. The influence of the chelating phosphine ligand on structural characteristics of these compounds has been investigated. Complexes of the type (2) and (3) can be synthesised from type (6) complexes. Detailed studies on the behaviour of a series of complexes of the type [Ni(diphos)(_2)](Y)(_2) (2) and [Ni(diphos)(Y)(_2)] (3) {Y = OTs and OTf), and the equilibrium between them in solution have been undertaken. The complex [Ni(dppe)(-2)](OTs)(_2) has been characterised by X-ray crystallography.