The novel complexes [M(L–L)(H₂dmsucc-S,S′)][M = Ni, Pd or Pt; L–L = 1,2-bis(diphenyl-phosphino)ethane, dppe; M = Pd or Pt, L–L = 1-diphenylarsino-2-diphenylphosphinoethane, dadpe; H₄dmsucc = dimercaptosuccinic acid) and [Ni(dppe)(Hmsucc)](H₃msucc = mercaptosuccinic acid) have been prepared and characterised. The reactions of sodium 2,3-dimercaptopropanesulfonate, sodium 1-thio-β-D-glucopyranosate and dithiothreitol with [M(L–L)X₂](M = Ni, Pd or Pt; X = Cl or Br) were investigated by ³¹P-{¹H} NMR spectroscopy. The ³¹P NMR chemical shifts, ³¹P–³¹P and ¹⁹⁵Pt–³¹P coupling constants, and the conformations of chelate rings (derived from ¹H–¹H coupling constants) in mixed-ligand product from these reactions are discussed. The use of these thiolate ligands has enabled solubilisation of several complexes containing dppe or dadpe ligands in aqueous media. The cytotoxicities of dppe and dadpe complexes of Pd and Pt with dimercaptosuccinic acid were investigated.