Mixed-ligand hydrazine complexes (M(CO)(RNHNH2)P-4](BPh4)(2) (1, 2) [M = Ru, Os; R = H, CH3, C6H5; P = P(OEt)(3)] with carbonyl and triethyl phosphite were prepared by allowing hydride [MH(CO)P4)BPh4 species to react first with HBF(4)center dot Et2O and then with hydrazines. Depending on the nature of the hydrazine ligand, the oxidation of [M(CO)(RNHNH2)P-4](BPh4)(2) derivatives with Pb(OAc)(4) at -30 degrees C gives acetate [M(kappa(1)-OCOCH3)(CO)P-4]BPh4 (3a), phenyldiazene (M(CO)(C6H5N=NH)P-4](BPh4)(2) (3c, 4c), and methyldiazene [M(CO)(CH3N=NH)P-4](BPh4)(2) (3b, 4b) derivatives. Methyldiazene complexes 3b and 4b undergo base-catalyzed tautomerization of the CH3N=NH ligand to formalclehyde-hydrazone NH2N=CH2, giving the [M(CO)(NH2N=CH2)p(4)](BPh4)(2) (5, 6) derivatives. Complexes 5 and 6 were characterized spectroscopically and by the X-ray crystal structure determination of the [Ru(CO)(NH2N=CH2){P(OEt)(3)}(4)](BPh4)(2) (5) derivative. Acetone-hydrazone [M(CO){NH2N=C(CH3)(2)}P-4](BPh4)(2) (7, 8) complexes were also prepared by allowing hydrazine [M(CO)(NH2NH2)P-4](BPh4)(2) derivatives to react with acetone.