A series of complexes of the type [(L-L)Pd(alkene)], where (L-L) is a diphosphine ligand and the alkene is dibenzylideneacetone (dba), benzoquinone or tetracyanoethene have been synthesised. These complexes have been evaluated as pre-catalysts in the methoxycarbonylation of ethene (80 C, 10 bar CO/ethene, MeOH, 20 MeSO(_3)H). Where (L-L) is the diphosphine l,2-bis(di-tert-butylphosphinomethyl)benzene and the alkene is dba, rates in excess of 40,000 moles methyl propionate/mole Pd/hr are obtained at 99.6% selectivity. The catalytic activity and selectivity is found to critically depend on the nature of the diphosphine ligand, with l,2-bis(diphenylphosphinomethyl)benzene complexes giving rise to high molecular weight, perfectly alternating CO/ethene co-polymer. Several of the pre-catalyst complexes have been characterised by single crystal X-ray diffraction, and factors common to both methyl propionate selective catalysts and co-polymer selective catalysts have been identified. The phosphine ligands chosen in this study all readily form complexes with Pd(_2)(dba)(_3). The complexes have the formula [(P-P)Pd(dba)] where the bidentate phosphine binds as a chelate ligand to a single palladium atom. The environment around the palladium is essentially trigonal planar, with only small dihedral angles between P(-2)Pd and PdC(_2) planes observed. Detailed studies of the reaction of the complexes [(L-L)Pd(alkene)] with methanesul- phonic acid have been undertaken. The reaction product is shown to depend on the nature of the diphosphine ligand and the alkene. The catalytic activity is discussed with reference to the ability of the reaction products to enter the catalytic cycle. The coordination chemistry of several diphosphines when mixed with Pd(II) salts has been studied. Several complexes have been characterised by X-ray crystallography and the features related to the observed catalytic activity. The reaction chemistry has also been explored and related to the observed catalysis. A mechanism which takes account of all the results reported in this thesis is presented.