The methylhydrazine complex [Ru(NH2NHMe)(PyP) 2]Cl(BPh4) (PyP = 1-[2-(diphenylphosphino)ethyl]pyrazole) was synthesised by addition of methylhydrazine to the bimetallic complex [Ru(μ-Cl)(PyP)2]2(BPh4)2. The methylhydrazine ligand of the ruthenium complex has two different binding modes: side-on (η2-) when the complex is in the solid state and end-on (η1-) when the complex is in solution. The solid-state structure of [Ru(PyP)2-(NH2NHMe)]Cl(BPh4) was determined by X-ray crystallography. 2D NMR spectroscopic experiments with 15N at natural abundance confirmed that in solution the methylhydrazine is bound to the metal centre by only the -NH2 group and the ruthenium complex retains an octahedral conformation. Hydrazine complexes [RuCl(PyP) 2(η1-NH2NRR′)]OSO2CF 3 (in which R = H, R′ = Ph, R = R′ = Me and NRR′ = NC5H10) were formed in situ by the addition of phenylhydrazine, 1,1-dimethylhydrazine and N-aminopiperidine, respectively, to a solution of the bimetallic complex [Ru(μ-Cl)(PyP)2] 2-(OSO2CF3)2 in dichloromethane. These substituted hydrazine complexes of ruthenium were shown to exist in an equilibrium mixture with the bimetallic starting material.