The reaction of Rh(NO3)3.cntdot.2H2O with PPh3 and N2H4.cntdot.H2O in MeOH gave the dinuclear carbopentazane complex [Rh2(PPh3)4{(NH2NH)2CH2}](NO3)2 (I), which was characterized by multinuclear NMR measurements and x-ray diffraction (monoclinic, space group P21/n, R = 0.041). It has a dipos., dinuclear cation in which the two bis(triphenylphosphine)rhodium units are bridged by the tetradentate carbopentazane ligand. The Rh atoms have the expected square-planar geometry, with Rh-P and Rh-N distances of 2.225(3)-2.253(3) and 2.130(6)-2.166(6) .ANG., resp. Expts. aimed at elucidating the mechanism of formation of I indicated that the formation of the NCH2N linkage occurs via acid-catalyzed nucleophilic addn. of free N2H4 to the CH2 group of an NH2N:CH2 ligand coordinated to RhI, and unambiguous multinuclear NMR (13C, 15N and 31P) and mass spectrometric evidence was obtained for monomeric [cyclic] [Rh(PPh3)2(NH2NHCH2NHNH2)]+ (II), which readily rearranges to form I. Related expts. also provided evidence for the formation of [cyclic] [Rh(PPh3)2(NH2NHCHMeNHNH2)]+, which also rearranges to give a dinuclear complex, [Rh2(PPh3)4{(NH2NH)2CHMe}]2+.