N,N′-Diphenylguanidine reacted with the hydrido complexes [MH 2 (CO)(PPh 3 ) 3 ] (M = Ru or Os) and mer-[IrH 3 (PPh 3 ) 3 ] in boiling toluene to afford the complexes [MH{PhNC(NH 2 )NPh}(CO)(PPh 3 ) 2 ] and [IrH 2 {PhNC(NH 2 )NPh}(PPh 3 ) 2 ] in which the N,N′-diphenylguanidinate anions adopt a symmetrical chelate co-ordination mode. Under similar conditions the trifluoroacetato complexes [M(O 2 CCF 3 ) 2 (CO)(PPh 3 ) 2 ] afforded salts [M{PhNC(NH 2 )NPh}{PhNC(NH 2 )NPh ·H}(CO)(PPh 3 ) 2 ][O 2 CCF 3 ] containing one chelate N,N′-diphenylguanidinate anion and one monodentate N,N′-diphenylguanidine ligand. Spectroscopic evidence indicates that the trifluoroacetate anions are hydrogen bonded to one or more of the N-bound hydrogens.