Ruthenium Nitrosyl Complexes with N-Heterocyclic Ligands.

A new route was developed for preparing a series of trans nitrosyl complexes of general formula trans-[Ru(NH(3))(4)L(NO)](BF(4))(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 nu(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(NH(3))(4)L(NO)](3+) + 2OH(-) right arrow over left arrow trans-[Ru(NH(3))(4)L(NO(2))](+) + H(2)O, 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)(NH(3))(4)L(NO(+))](3+) + e(-) right arrow over left arrow trans-[Ru(II)(NH(3))(4)L(NO(0))](2+) was observed in the range -0.45 to 1.20 V for all the nitrosyl complexes. Linear correlations are observed in plots of nu(NO) versus E(1/2) and of E(1/2) versus summation operatorE(L) showing that the oxidizing strength of the coordinated NO increases with increase in L pi-acidity. The crystal structure analysis of trans-[Ru(NH(3))(4)nicNO](2)(SiF(6))(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 Å, thus confirming the presence of the Ru(II)-NO(+) moiety in the nitrosyl complexes studied.