The seven-coordinate halocarbonyl complexes, [MXY(CO)(3)(NCMe)(2)] (M = Mo or W; X, Y = halide) and their derivatives have been investigated in several types of homogeneous catalytic reactions. These include, the alkene metathesis polymerisation of norbornadiene using either, [MX2(CO)(3)(NCMe)(2)] (M = Mo or W; X = Br or I), [MI2(CO)(3)(L'-P, P')] {M = Mo or W; L' = MeC(CH2PPh2)(3)} or [MXY(CO)(L or L'-P, P')(eta(2)-RC2R')] {X,Y = Cl, Br or I; L = PhP(CH2CH2PPh2)(2); R,R' = Me, Ph). The isolation and characterisation of a key intermediate, [WI2(CO)(2)(eta(4)-nbd)] (1) (nbd = norbornadiene) is also described. The development of water soluble polymerisation catalysts using water-solubilising pyridine ligands, and the biphasic polymerisation of styrene using [WI2(CO)(3)(NC5H4CO2Na-4)(2)], ZrCl4, sodium lauryl sulphate and sodium hydrogen phosphate is also discussed. The room temperature polymerisation of phenylacetylene using [WX2(CO)(3)(NCMe)(2)] (X = Cl, I), [WI2(CO)(3)(NCMe)L] (L = PPh3, AsPh3) or [WBr2(CO)(NCMe)(eta(2)-HC2Ph)(2)] (4) is also described, and a mechanism is proposed for these polymerisation reactions.