The crystal structure of trans-[Rh(CO)(PPh3)(OClO3)] has been determined, an it has been established that the perchlorate group is easily displaced by substituted hydrazines to give trans-[Rh(CO)(PPh3)2L]ClO4 (L = NH2NH2-xRx; x = 1 or 2; R = Me or Ph). X- Ray crystallography shows that for all these complexes the unsymmetrically substituted hydrazine co-ordinates to rhodium via the NH2 group.