Tris(pyrazolyl)borate aryldiazenido complexes [RuTpLL'(ArN2)](BF4)(2) (1-3) [Ar = C6H5, 4-CH3C6H4; Tp = hydridotris(pyrazolyl)borate; L = P(OEt)(3) or PPh(OEt)(2), L' = PPh3; L = L' = P(OEt)(3)] were prepared by allowing dihydrogen [RuTp(eta(2)-H-2)LL'](+) derivatives to react with aryldiazonium cations. Spectroscopic characterization (IR, N-15 NMR) using the N-15-labeled derivatives strongly supports the presence of a linear [Ru]-Nequivalent toN-Ar aryldiazenido group. Hydrazine complexes [RuTp(RNHNH2)LL']BPh4 (4-6) [R = H, CH3, C6H5, 4-NO2C6H4; L = P(OEt)(3) or PPh(OEt)(2), L' = PPh3; L = L' = P(OEt)(3)] were also prepared by reacting the [RuTp(eta(2)-H-2)LL'](+) cation with an excess of hydrazine. The complexes were characterized spectroscopically (IR and NMR) and by X-ray crystal structure determination of the [RuTp(CH3NHNH2){P(OEt)(3)}(PPh3)]BPh4 (4d) derivative. Tris(pyrazolyl)borate aryldiazene complexes [RuTp(ArN=NH)LL']BPh4 (7-9) (Ar = C6H5, 4-CH3C6H4) were prepared following three different methods: (i) by allowing hydride species RuHTpLL' to react with aryldiazonium cations in CH2Cl2; (ii) by treating aryldiazenido [RuTpLL'(ArN2)](BF4)(2) with LiBHEt3 in CH2Cl2; (iii) by oxidizing arylhydrazine [RuTp(ArNHNH2)LL']-BPh4 complexes with Pb(OAc)(4) in CH2Cl2 at -30 degreesC. Methyldiazene complexes [RuTp(CH3N=NH)LL']BPh4 were also prepared by the oxidation of the corresponding methylhydrazine [RuTp(CH3NHNH2)LL']BPh4 with Pb(OAc)(4).