A series of complexes of the type [Rh(diphos)(C(_7)H(_8))](BF(_4)) (1) have been synthesised and two of these species [diphos = Bu(^1)(_2)P(CH(_2))(_2)PBu(^1)(_2), Cy(_2)P(l,2-trans-cyclopentane)PCy(_2)] have been characterised by X-ray crystallography. The influence of the chelating diphosphine on the structural characteristics and NMR parameters of these compounds has been investigated. Complexes of the type (1) are active catalysts for the hydroformylation of l-hexene(120ºC, 450psi 2:1 H(_2):C0). However, they are inactive for the related hydroesterification process and this is attributed to the stability of [Rh(diphos)(CO)(_2)]^ under the reaction conditions. Detailed studies of the reactions of type (1) complexes with H(_2) have been undertaken. The products from these reactions are dependent on both the diphosphine and the solvent employed. In THF and CD(_2)Cl(_2), dimeric hydrides were observed, whilst in CDCl(_3) hydrido-chloro-complexes of the type [Rh(_z)(diphos)(_2)(|µCl)H(_2)](^2+) were formed. In the former solvents, three types of hydrides have been identified by NMR studies, namely [Rh(_2)(diphos)(_2)H(_6)], [Rh(_2)(diphos)(_2)H(_4)] and [Rh(_2)(diphos)(_2)H(_4)](^2+). The oxidative-addition reactions of alkyl-, acyl- and formyl-halide with Rh(I) complexes have been investigated. The reaction with ClCO(_2)Me with [Rh(dppe)Cl](_2) and [Rh(dppe)](_2)(BF(_4))(_2) led to the formation of diphosphine-rhodium(III)-halide species (H(_3)NOH)[Rh(dppe)Cl(_4)] and [Rh(_2)(dppe)(_2)(µ-Cl)(_3)Cl(_2)](BF(_4)) respectively. Alternative routes to these complexes have been investigated starting from RhCl(_3).3H(_2)O/diphosphines and a series of the novel halide bridged dimers have been characterised, including by X-ray crystallography and their reaction chemistry explored. In the synthesis of the cationic rhodium complexes from neutral chloro-complexes with AgBF(_4), the novel silver complexes [Ag(_2)(µ-Bu(^t)(_2)P(CH(_2))(_3)PBu(^t)(_2))(_2)](BF(_4))(_2), [{Ag(H(_2)O)}(_2)(µ-Bu(^t)(_2)P(CH(_2))(_2)PBu(^t)(_2))](BF(_4))(_2), and [Ag(C(_6)D(_6))(_3)BF(_4)] have been isolated as by-products and characterised by X-ray crystallography.