THE ROLE OF THE 16-ELECTRON [(ETA-5-C5ME5)RU(NO)] TRANSIENT IN THE FORMATION OF DINUCLEAR COMPLEXES AND IN OXIDATIVE ADDITION-REACTIONS

The formation of [Cp*Ru(mu-NO)]2 (2) from the treatment of Cp*Ru(NO)Cl2 (1) with Zn dust in EtOH is preceded by the formation of an intermediate complex [Cp*Ru(mu-NO)Cl]2 (4) containing a formal Ru-Ru single bond (Cp* = eta-5-C5Me5). Complex 4 is fully characterized, including a single-crystal X-ray structure: monoclinic space group P2(1)/n, a = 8.272 (3) angstrom, b = 14.722 (5) angstrom, c = 9.863 (3) angstrom, beta = 107.42 (2)degrees, Z = 4, R(w) = 5.28%, based on 1301 observed data (F > 4.O-sigma(F)). The structure shows a centrosymmetric trans geometry with bridging nitrosyl ligands, terminal chloride ligands, and a Ru-Ru distance of 2.684 (2) angstrom. Purified complex 4 reacts further with Zn dust in EtOH to give 2 quantitatively. Complex 4 is formed together with Cp*Ru(NO)(CH2Cl)Cl (6) in the reaction of Cp*Ru(NO)Ph2 (5a) with CH2Cl2. The fact that complex 4 is formed in high yield from the thermolysis of an equimolar mixture of 5a and 1 in ethanol suggests that any [Cp*Ru(NO)] transients produced in the Zn reaction are efficiently trapped to complex 4 by excess 1. Crossover experiments involving 5a and Cp*Ru(NO)(p-tolyl)2 (5b) help verify that the generation of the 16-electron [Cp*Ru(NO)] species is the first process to occur when Cp*Ru(NO)(aryl)2 complexes are thermalized in chlorinated and non-chlorinated solvents. Thermolysis of 5a in 1,2-dichloroethane gives complex 4 and ethylene, apparently through the generation of an unstable beta-chloroethyl complex which decomposes to ethylene and dichloride complex 1; the absence of 1 in the final reaction residue is attributed to its consumption by [Cp*Ru(NO)] transients, leading to 4 as the only observed organometallic product.