A new route was developed for preparing a series of trans nitrosyl complexes of general formula trans-[Ru(NH3)4(L)(NO)](BF4)(3), where L = imidazole : L-histidine, pyridine, or nicotinamide. The complexes have been characterized by elemental analysis, molar conductance measurements, UV-visible, infrared, proton nuclear magnetic, and electron paramagnetic resonance spectroscopies, and electrochemical techniques. The compounds possess relatively high v(NO) stretching frequencies indicating that a high degree of positive charge resides on the coordinated nitrosyl group. The nitrosyl complexes react with OH-according to the equation trans-[Ru(NH3)(4)L(NO)](3+) + 20H(-) reversible arrow trans-[Ru(NH3)(4)L(NO2)](+) + H2O, with a K-eq (at 25.0 degrees C in 1.0 mol/L NaCl) of 2.2 x 10(5), 5.9 x 10(7), 9.7 x 10(10), and 4.6 x 10(13) L-2 mol(-2) for the py, nic, imN, and L-hist complexes, respectively. Only one redox process attributed to the reaction [Ru-II(NH3)(4)L(NO+)](3+) + e(-) reversible arrow trans-[Ru-II(NH3)(4)L(NO0)](2+) was observed in the range -0.45 to 1.20 V for all the nitrosyl complexes. Linear correlations are observed in plots of v(NO) versus E-1/2 and of E-1/2 versus Sigma E-L showing that the oxidizing strength of the coordinated NO increases with increase in L pi-acidity. The crystal structure analysis of trans-[Ru(NH3)(4)nicNO](2)(SiF6)(3) shows that the mean Ru-N-O angle is very close to 180 degrees (177 +/- 1 degrees) and the mean N-O distance is 1.17 +/- 0.02 Angstrom, thus confirming the presence of the Ru-II-NO+ moiety in the nitrosyl complexes studied.