The diazoalkane complexes [Ru(eta(5)-C5Me5)(N(2)CAr1Ar2){P(OR)(3)}L]BPh4 (1-4) [R = Me, L = P(OMe)(3) (1); R = Et, L = P(OEt)(3) (2); R = Me, L = PPh3 (3); R = Et, L = PPh3 (4); Arl = Ar2 = Ph (a); Arl = Ph, Ar2 = p-tolyl (b); ArlAr2 = C12H8 (c); Arl = Ph, Ar2 = PhC(O) (d)] and [Ru(eta(5)-C5Me5){N2C(C12H8)}{PPh(OEt)(2)}(PPh3)]BPh4 (5c) were prepared by allowing chloro-compounds RuCl(eta(5)-C5Me5)[P(OR)(3)]L to react with the diazoalkane Ar1Ar2CN(2) in the presence of NaBPh4. Treatment of complexes 1-4 with H2O afforded 1,2-diazene derivatives [Ru(eta(5)-C5Me5)(eta(2)-NH=NH){P(OR)(3)}L]BPh4 (6-9) and ketone Ar1Ar2CO. A reaction path involving nucleophilic attack by H2O on the coordinated diazoalkane is proposed and supported by density functional theory calculations. The complexes were characterized spectroscopically (IR and H-1, P-31, C-13, N-15 NMR) and by X-ray crystal structure determination of [Ru(eta(5)-C5Me5)(N2CC12H8){P(OEt)(3)}(2)]BPh4 (2c) and [Ru(eta(5)-C5Me5)(eta(2)-NH=NH){P(OEt)(3)}(2)]BPh4 (7).