cis- and trans-nitrosyltetraammineruthenium(II). Spectral and electrochemical properties and reactivity

A synthetic route was developed for the preparation of trans-[Ru(NH3)(4)(NO)X](n+), where X = isonicotinamide (isn), pyrazine (pyz) or sulfite, and cis-[Ru(NH3)(4)(NO)(NO2)](2+). The complexes have been characterized by elemental analysis, UV/VIS, infrared, H-1 NMR and ESR spectroscopies, molar conductance measurements and cyclic voltammetry. All showed v(NO) in the range characteristic of metal-co-ordinated NO+ and do not exhibit any ESR signal, consistent with the formulation of Ru-II-NO+. The equilibrium constants K-eq for the reaction trans-[Ru(NH3)(4)(NO)X](3+) + 20H(-) reversible arrow trans-[Ru(NH3)(4)(NO2)X](+) + H2O are 2.5 x 10(8) and 6 x 10(8) dm(6) mol(-2) for X = isn or pyz. Cyclic voltammograms of the complexes in aqueous solution exhibited reversible one-electron waveforms in the potential range -0.13 to -0.38 V vs. SCE, which were assigned to the [Ru(NH3)(4)(No)X](n+) --> [Ru(NH3)(4)(NO)X]((n-1)+) process. Nitric oxide and trans-[Ru(NH3)(4)(H2O)X](2+) are the final products of the reaction between Eu-II and trans-[Ru(NH3)(4)(NO)X](3+), L = isn or pyz. Ab initio molecular orbital calculations performed for trans-[Ru(NH3)(4)(NO)(pyz)](3+) and trans-[Ru(NH3)(4)(NO)(pyz)](2+), and the products of the trans-[Ru(NH3)(4)(NO)(pyz)](3+) one-electron electrochemical or chemical reduction, strongly suggest the added electron is localized mainly on the nitrosyl ligand. A correlation was observed between v(NO) and E-1/2 for the reversible reduction wave. These results indicate that the nitric oxide reduction is facilitated by strong pi-acceptor ligands trans to the NO. Nitric oxide and trans-[Ru(NH3)(4)(H2O)X](3+) were formed when solutions containing trans-[Ru(NH3)(4)(NO)X](3+) were irradiated in the range 310-370 nm.