The reaction of cis-[RuCl2(PP)2] (PP = depe, dmpe) with hydrazine afforded end-on bound ruthenium(II) hydrazine complexes. Treatment of the hydrazine complexes with strong base afforded the side-on bound ruthenium(0) diazene complexes cis-[Ru(?2-NH - NH)(PP) 2]. Treatment of cis-[Ru(?2-NH - NH)(depe) 2] with weak acid under chloride-free conditions afforded the side-on bound hydrazine complex cis-[Ru(?2-N2H 4)(depe)2]2+. These are the first reported side-on bound diazene and hydrazine complexes of ruthenium, and they have been characterized by NMR spectroscopy (1H, 31P, 15N) and by X-ray crystallography. The interconversion between the ruthenium diazene and the ruthenium hydrazine by acid?base treatment was reversible. © 2010 American Chemical Society.