Chapter 1 reviews the literature concerning the principal synthetic routes to hydroxyphosphine transition-metal complexes and their reactions, which have led to the formation of some unusual diphosphoxane derivatives. The preparation and characterization of aminophosphine complexes of chromium, molybdenum, and tungsten are presented in Chapter 2. The phosphorus-nitrogen bond is readily cleaved by HC1 gas and other aqueous acids, to afford phosphorus-chlorine and phosphorus-oxygen bonded derivatives. The chromium and tungsten aminophosphine complexes have been found to be particularly useful for the preparation of chlorodiphenylphosphine derivatives. In addition their reactions with alcohols have also been investigated. The study of aminophosphine complexes has been extended to Pt(II) derivatives in Chapter 3. Cleavage of the phosphorus-nitrogen bond in these complexes is again observed to proceed smoothly with gaseous HC1 and aqueous HC1, but not with other aqueous acids. Treatment of the chlorodiphenylphosphine complex, cis-[PtC12(PPh2C1)2], with sodium sulphide afforded a four-membered metallacycle. A similar four-membered metallacycle was also obtained by refluxing the hydroxyphosphine complex, cis-[PtC12(PPh2OH)2], in toluene for 12h. Other reactions of the hydroxyphosphine complex with chlorodiphenylphosphine and dichlorophenyl- phosphine led to the formation of six-membered ring systems. The molecular structure of the complex [NHEt3] [Pt(C?CPh)(Ph2PO)2 (Ph2POH)] has been established by X-ray crystallography and reveals an unsymmetrical hydrogen-bridged system. Some of the complexes have been characterised via their 1,2-dithiolene derivatives. Treatment of cis-[PtC12(PNEt2Ph2)2] with diphenacyl sulphone in the presence of silver(I) oxide afforded the metallathietane-3, 3-dioxide complex, L2Pt-CHR-S (O)2-CHR (L=Ph2PNEt2; R = C(O)Ph). The final chapter describes the preparation of some Rh(I) and Ir(I) aminophosphine complexes. Their oxidative addition reactions with methyl iodide, tetracyanoethylene, dioxygen, and HC1 gas have been investigated. In addition, some interesting metal-mercury bonded complexes were prepared by the treatment of trans-[IrC1 (CO) (PNEt2Ph2)2] with mercuric halides, HgX2 (X=C1, Br or I). Satellite peaks due to mercury-199 coupling (I=1/2, 16.9%) were readily observed in the 31P-{1H} n.m.r. spectra.