Reaction of OsH2P4 [P = P(OEt)(3), PPh(OEt)(2), PPh2OEt] with methyl triflate followed by the treatment with hydrazines gave the [OsH(RNHNH2)P-4]BPh4 (1-3) (R = H, CH3, C6H5, 4-NO2C6H4) derivatives. Instead, the reaction of OsH2P4 first With methyl triflate, then with triflic acid, and finally with an excess of the appropriate hydrazine afforded the bis(hydrazine) [Os(RNHNH2)(2)P-4](BPh4)(2) (4, 5) (R = H, CH3, C6H5) complexes. Also the [Os(NH2NH2){P(OEt)(3)}(5)](BPh4)(2) (7) derivative was prepared. All the hydrazine complexes were fully characterized by IR and H-1 and P-31 NMR spectra, and a single-crystal X-ray structure determination of the complex [Os(NH2NH2)(2){P(OEt)(3)}(4)](BPh4)(2) . C2H5OH (4a) is reported. The compound crystallizes in the space group P2(1)/c with a = 20.550(4) Angstrom, b = 19.663(4) Angstrom, c = 20.843(4) Angstrom, 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(NH2NH2)(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)NH2}{P(OEt)(3)}(4)](BPh4)(2) (8, 9) (R = H, CH3; R1 = CH3, 4-CH3C6H4) were prepared by allowing nitrile complexes [Os(R1CN)(2)P-4](BPh4)(2) to react with hydrazine NH2NH2 or methylhydrazine CH3NHNH2. Reaction of complexes containing substituted hydrazine ligands of the type [OsH(RNHNH2)P-4]BPh4 and [Os(RNHNH2)(2)P-4](BPh4)(2) [P = P(OEt)(3); R = CH3, C(6)H5] 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]BPh4 (10) and [Os(RN=NH)(2)P-4](BPh4)(2) (11) derivatives. The first bis(methyldiazene) complex [Os(CH3N=NH)(2){P(OEt)(3)}(4)](BPh4)(2) (11b) was thus prepared.