Hydrazine complexes [Fe(R¹NHNH₂)(R²CN)L₄][BPh₄]₂ 1–3 and [FeH(R¹NHNH₂)L₄]BPh₄ 4 [R¹ = H, Me, Ph, 4-MeC₆H₄, 4-ClC₆H₄, 4-O₂NC₆H₄ or Me₂NNH₂; R² = 4-MeC₆H₄, Me or Ph; L = P(OEt)₃, P(OMe)₃ or PPh(OEt)₂] were prepared by reacting nitrile complexes [Fe(R²CN)₂L₄][BPh₄]₂ and [FeH(R²CN)L₄]BPh₄ with an excess of the appropriate hydrazine. Their formulation and geometry in solution were established by infrared and ¹H and ³¹P NMR spectroscopy. Amidrazone derivatives [Fe{η²-NHC(R²)NR¹NH₂}L₄][BPh₄]₂ [R² = 4-MeC₆H₄, Me or Ph; R¹ = H or Me; L = P(OEt)₃ or P(OMe)₃] were obtained by allowing nitrilehydrazine complexes to react in solution and their formation, involving a nucleophilic attack of the hydrazine on the cyanide carbon atom of the R²CN ligand, is also discussed. Reaction of the arylhydrazine complexes 1–4 with Pb(O₂CMe)₄ at –30 °C in CH₂Cl₂ resulted in selective oxidation of the arylhydrazine ligand affording the aryldiazene [Fe(R¹NNH)(R²CN)L₄][BPh₄]₂ and [FeH(R¹NNH)L₄]BPh₄ 10 [R¹ = aryl; L = P(OEt)₃ or P(OMe)₃] derivatives. Treatment of the arylhydrazine [Fe(R¹NHNH₂)(R²CN){P(OEt)₃}₄][BPh₄]₂ 1 (R¹ = Ph or 4-MeC₆H₄) with an excess of NEt₃ resulted in a complicated reaction giving the five-co-ordinate aryldiazenido derivatives [Fe(R¹N₂){P(OEt)₃}₄]BPh₄.