Reaction of OsH(2)P(4) [P = P(OEt)(3), PPh(OEt)(2), PPh(2)OEt] with methyl triflate followed by the treatment with hydrazines gave the [OsH(RNHNH(2))P(4)]BPh(4) (1-3) (R = H, CH(3), C(6)H(5), 4-NO(2)C(6)H(4)) derivatives. Instead, the reaction of OsH(2)P(4) first with methyl triflate, then with triflic acid, and finally with an excess of the appropriate hydrazine afforded the bis(hydrazine) [Os(RNHNH(2))(2)P(4)](BPh(4))(2) (4, 5) (R = H, CH(3), C(6)H(5)) complexes. Also the [Os(NH(2)NH(2)){P(OEt)(3)}(5)](BPh(4))(2) (7) derivative was prepared. All the hydrazine complexes were fully characterized by IR and (1)H and (31)P NMR spectra, and a single-crystal X-ray structure determination of the complex [Os(NH(2)NH(2))(2){P(OEt)(3)}(4)](BPh(4))(2).C(2)H(5)OH (4a) is reported. The compound crystallizes in the space group P2(1)/c with a = 20.550(4) Å, b = 19.663(4) Å, c = 20.843(4) Å, beta = 99.84(9) degrees, and Z = 4. The coordination around the osmium atom is octahedral and the orientation of the ligands in the [Os(NH(2)NH(2))(2){P(OEt)(3)}(4)](2+) cation is determined by several strong hydrogen bonds involving hydrazine nitrogen and phosphite oxygen atoms. Amidrazone complexes [Os{eta(2)-NH=C(R1)N(R)NH(2)}{P(OEt)(3)}(4)](BPh(4))(2) (8, 9) (R = H, CH(3); R1 = CH(3), 4-CH(3)C(6)H(4)) were prepared by allowing nitrile complexes [Os(R1CN)(2)P(4)](BPh(4))(2) to react with hydrazine NH(2)NH(2) or methylhydrazine CH(3)NHNH(2). Reaction of complexes containing substituted hydrazine ligands of the type [OsH(RNHNH(2))P(4)]BPh(4) and [Os(RNHNH(2))(2)P(4)](BPh(4))(2) [P = P(OEt)(3); R = CH(3), C(6)H(5)] with Pb(OAc)(4) at -30 degrees C results in the selective oxidation of the hydrazine affording the corresponding diazene [OsH(RN=NH)P(4)]BPh(4) (10) and [Os(RN=NH)(2)P(4)](BPh(4))(2) (11) derivatives. The first bis(methyldiazene) complex [Os(CH(3)N=NH)(2){P(OEt)(3)}(4)](BPh(4))(2) (11b) was thus prepared.