Hydrazine complexes [RuH(R¹NHNH₂)L₄]BPh₄ 1–3, [Ru(R¹NHNH₂)₂L₄][BPh₄]₂ 4–6 [R¹ = H, Me, Ph, 4-MeC₆H₄ or 4-O₂NC₆H₄; L = P(OEt)₃, PPh(OEt)₂ or P(OMe)₃] were prepared by allowing the hydride species [RuH₂L₄] to react, first with triflic acid (CF₃SO₃H) and then with an excess of the appropriate hydrazine. The derivatives [RuH(Me₂NNH₂){P(OEt)₃}₄]BPh₄ 1f, [Ru(η¹-OSO₂CF₃)(Me₂NNH₂){P(OEt)₃}₄]BPh₄ 9 and [Ru(η²-Ph-CONHNH₂)L₄][BPh₄]₂ 7,8 [L = P(OEt)₃ or PPh(OEt)₂] were also obtained. The formulation and geometry in solution of the compounds were established by infrared and ¹H and ³¹P NMR spectroscopy. The reaction of the bis(nitrile) complexes [Ru(R²CN)₂L₄][BPh₄]₂ [R² = Me or MeC₆H₄; L = P(OEt)₃ or PPh(OEt)₂] with hydrazines depends not only on the experimental conditions, but also on the nature of the phosphite and the hydrazine used. Thus, nitrilehydrazine [Ru(R¹NHNH₂)(R²CN)L₄][BPh₄]₂ 10–13 [L = P(OEt)₃ or PPh(OEt)₂] or amidrazone derivatives [Ru{η²-NHC(R²)N(R¹)NH₂}{P(OEt)₃}₄][BPh₄]₂ 14,15 (R¹ = H or Me) were obtained together with the bis(hydrazine) compounds [Ru(R¹NHNH₂)₂L₄][BPh₄]₂. Reaction of the arylhydrazine complexes 1–6 and 10–13 with Pb(O₂CMe)₄ at –30 °C in CH₂Cl₂ resulted in selective oxidation of the arylhydrazine ligand giving the aryldiazene derivatives [RuH(R¹NNH)L₄]BPh₄, [Ru(R¹NNH)₂L₄][BPh₄]₂ and [Ru(R¹NNH)(R²CN)L₄][BPh₄]₂ (R¹ = Ph, 4-MeC₆H₄ or 4-O₂NC₆H₄). Treatment of hydrazine NH₂NH₂ and methylhydrazine MeNHNH₂ complexes 1–6 with Pb(O₂CMe)₄, instead, afforded the acetate [Ru(η²-O₂CMe)L₄]BPh₄ derivatives which were characterised by a crystal structure determination of [Ru(η²-O₂CMe){P(OEt)₃}₄]BPh₄. The co-ordination of ruthenium is distorted octahedral with approximate C2_v symmetry and the acetate is bidentate.