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.

Substitution of MeCN ligands in the activated cluster Os3(CO)10(MeCN)2 by the unsaturated diphosphine ligands (Z)-Ph2PCH=CHPPh2 (cDPPEn) or 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) proceeds rapidly at room temperature to furnish the ligand-bridged cluster 1,2-Os3(CO)10(P-P) (P-P represents cDPPEn or bpcd). Heating 1,2-Os3(CO)10(P-P) leads to the formation of the thermodynamically more stable chelating isomer 1,1-Os3(CO)10(P-P). Each compound of Os3(CO)10(P-P) has been characterized by x-ray diffraction, IR, 31P NMR and 1H NMR. Ligand isomerization kinetics have been investigated by UV-VIS and 31P NMR (for cDPPEn) or 1H NMR (for bpcd) spectroscopies. The isomerization mechanism is discussed based on the activation parameters and CO inhibition (for cDPPEn) or ligand trapping experiments (for bpcd). Thermolysis of 1,1-Os3(CO)10(bpcd) in refluxing toluene gives the hydrido cluster HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and the benzyne cluster HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)]. Photolysis of 1,1-Os3(CO)10(bpcd) using near UV light affords HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] as the sole product. HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been characterized in solution by IR and NMR spectroscopies. Furthermore its molecular structure has been determined by X-ray crystallography. Reversible C-H bond formation in HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] is demonstrated by ligand trapping studies to give 1,1-Os3(CO)9L(bpcd) (where L = CO, phosphine) via the unsaturated intermediate 1,1-Os3(CO)9(bpcd). The kinetics for reductive coupling in HOs3(CO)9[γ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] and DOs3(CO)9[μ-(PPh2-d10)C=C{P(Ph-d5)(C6D4)}C(O)CH2C(O)] in the presence of PPh3 give rise to a kH/kD value of 0.88, whose magnitude supports the existence of a preequilibrium involving the hydride(deuteride) cluster and a transient arene-bound Os3 species that precedes the rate-limiting formation of 1,1-Os3(CO)9(bpcd). Strong proof for the proposed hydride(deuteride)/arene preequilibrium has been obtained from photochemical studies employing the isotopically labeled cluster 1,1-Os3(CO)10(bpcd-d4ortho), whose bpcd phenyl groups each contain one ortho hydrogen and deuterium atom. Equilibrium and kinetic isotope effects in the orthometallation step has been determined by 1H NMR in photochemical studies. Kinetics for the transformation from HOs3(CO)9[μ-(PPh2)C=C{PPh(C6H4)}C(O)CH2C(O)] to HOs3(CO)8(μ3-C6H4)[μ2,η1-PPhC=C(PPh2)C(O)CH2C(O)] has been studied by UV-VIS spectroscopy for which the mechanism is discussed.