Triazenide [M(eta(2)-1 3-ArNNNAr)P-4]BPh4 [M = Ru, Os; Ar = Ph, p-tolyl; P = P(OMe)(3), P(OEt)(3), PPh(OEt)(2)] complexes were prepared by allowing triflate [M(K-2-OTf)P-4]OTf species to react first with 1,3-.ArN=NN(H)Ar triazene and then with an excess of triethylamine. Alternatively, ruthenium triazenide [Ru(eta(2)-1,3-ArNNNAr)P-4]BPh4 derivatives were obtained by reacting hydride [RuH(eta(2)-H-2)P-4](+) and RuH(kappa(1)-OTf)P-4 compounds with 1,3-diaryltriazene, The complexes were characterized by spectroscopy and X-ray crystallography of the [Ru(eta(2)-1,3-PhNNNPh){P(OEt)(3)}(4)]BPh4 derivative. Hydride triazene [OsH(eta(1)-1,3-ArN=NN(H)Ar)P-4]BPh4 [P = P(OEt)(3), PPh(OEt)2; Ar = Ph, p-tolyl] and [RuH{eta(1)-1,3-p-tolyl-N=NN(H)-p-tolyl}{PPh(OEt)(2)}(4)]BPh4 derivatives were prepared by allowing kappa(1)-triflate MH(kappa(1)-OTf)P-4 to react with 1,3-diaryltriazene. The [OS(kappa(1)-OTf){eta(1)-1,3-PhN=NN(H)Ph}{P(OEt)(3)}(4)]BPh4 intermediate was also obtained. Variable-temperature NMR studies were carried out using N-15-labeled triazene complexes prepared from the 1,3Ph N-15=(NN)-N-15(H)Ph ligand. Osmium dihydrogen [OsH(eta(2).H-2)P-4]BPh4 complexes [P = P(OEt)(3), PPh(OEt)(2)] react with 1,3-ArN=NN(H)Ar triazene to give the hydride-diazene [OsH(ArN=NH)P-4]BPh4 derivatives. The X-ray crystal structure determination of the (OsH(PhN=NH){PPh(OEt)(2)}(4)]BPh4 complex is reported. A reaction path to explain the formation of the diazene complexes is also reported.