The reaction between trans-[Ru-II(NO+)(NH3)(4)(L)](3+), L = ImN, IsN, Nic, P(OMe)(3), P(OEt)(3), and P(OH)(OEt)(2), and the Fe(III) species [Fe-III(TPPS)], metmyoglobin, and hemoglobin was monitored by UV-vis, EPR, and electrochemical techniques (DPV, CV). No reaction was observed when L = ImN, IsN, Nic, and P(OH)(OEt)(2). However, when L = P(OMe)(3) and P(OEt)(3), the reaction was quantitative and the products were trans-[Ru-III(H2O)(NH3)(4)(P(OR)(3))](3+) and [Fe-II(NO+)] species. Reaction kinetics data and DFT calculations suggest a two-step reaction mechanism with the initial formation of a bridged [Ru-(mu NO)-Fe] intermediate, which was confirmed through electrochemical techniques (E-0' = -0.47 V vs NHE). The calculated specific rate constant values for the reaction were in the ranges k(1) =1.1 to 7.7 L mol(-1) s(-1) and k(2) = 2.4 x 10(-3) to 11.4 x 10(-3) s(-1) for L = P(OMe)(3) and P(OEt)(3). The oxidation of the ruthenium center (Ru(II) to Ru(III)) containing the nitrosonium ligand suggests that NO can act as an electron transfer bridge between the two metal centers.