Reactions of chloro-complexes of palladium and platinum with dialkyl and diphenyl phosphonates and their sodium and silver salts give complexes of types [(I)–(VI)]; [M{(RO)2PO}2{(RO)2POH}2](I)(M = Pd and Pt), trans-[Pt{(RO)2PO}2L2](II)(L = Et3As, Et3P, and C5H5N), [MCl{(RO)2PO}{(RO)2POH}L](III), trans-[PtCl{(RO)2PO}L2](IV)(L = R3P and Et3As), [Pt{(RO)2PO}2(diphos)](V), [PdCl{(PhO)2PO}{(PhO2POH}](VI)(probably dimeric). The complex (IV)(R = Ph; L = Et3As) was also obtained by heating cis-[PtCl2–(Et3As)2] with [(I); M = Pt; R = Ph], and the complex [(III); M = Pt, R = Me; L = Et3P] was obtained from (VI) and triphenylphosphine. Reaction of [(I); M = Pt; R = Ph or Me] with 1,2-bisdiphenylphosphinoethane also gave [(V); R = Ph or Me]. Metathetical reactions with complex [(III); M = Pt; R = Ph; L = Bu3P] with sodium or silver salts of X gave the complexes [PtX{(PhO)2PO}(Bu3P)2](X = Br, I, NO2, NCO, N3, NCS, CN, NO3, and AcO) and the modes of co-ordination of the anionic ligands have been established. Structures have been derived from 1H and 31P n.m.r. and from i.r. spectra.