The ligands P-( N,N-dimethylaminomethyl)- P,P-diphenylphosphine oxide (dmpo) and P-( N,N-dimethylaminoethyl)- P,P-diphenylphosphine oxide (depo) were reacted with selected divalent cobalt and nickel salts. The resultant complexes were analysed and characterised. In all cases dmpo behaved as a bidentate ligand by coordinating through the amine and phosphine oxide groups while depo usually reacted in a monodentate manner. The complexes (MX 2) 3(dmpo) 4, where M = Co, Ni and X = Cl, Br, were shown to consist of two pentacoordinate M(dmpo) 2X + cations and one tetrahedral MX 4 2− anion. Reaction of equimolar quantities of CoCl 2 and depo resulted in the isolation of CoCl 3(H·depo). This complex was found to have a tetrahedral coordination sphere consisting of three chloride atoms and one phosphine oxide. It was suspected that a tautomerism type reaction produced the proton in H·depo +. The corresponding NiCl 3(H·depo)complex could only be obtained by the addition of hydrochloric acid. The reaction of equimolar amounts of CoCl 2, dmpo, and HCl yielded (H·dmpo) 2[CoCl 4]. The complex Co(dmpo) 2(SCN) 2 which contained two N-bonded thiocyanate groups was assigned a distorted octahedral coordination sphere. However, in Co(depo) 2(SCN) 2 the phosphine oxide groups did not bond and the coordination sphere had a distorted tetrahedral symmetry. The insoluble nature of Ni(depo) 2(ClO 4) 2·2H 2O resulted in inconclusive characterisation data. The complexes [M(dmpo) 2(mecn) 2](ClO 4) 2, mecn = methyl cyanide, were assigned an octahedral coordination sphere. Removal of methyl cyanide from this sphere resulted in complexes in which there was evidence for interaction between the perchlorate groups and the metal ions.