Monosubstituted diphenylphosphine derivatives of metal carbonyls are conveniently obtained by heating the carbonyl with one equivalent of the phosphine. Di- and tri-substituted derivatives of molybdenum hexacarbonyl can be prepared by replacing unsaturated hydrocarbon ligands by the phosphine. Fe2(CO)9 reacts with PPh2H to give Fe(CO)3(PPh2H)2. Infrared and 1H and 31P nuclear magnetic resonance spectra of the diphenylphosphine products permit structural assignments. A new type of sym–asym isomerism for disubstituted octahedral phosphine complexes is postulated.