Snapshots of the “breaking” of the H-H bond in the oxidative addition of H2 to a metal centre

Abstract

Three new monocationic molecular hydrogen complexes of ruthenium of the typetrans-[RuCl(η2-H2)(PP)2][BF4] (PP = bis-l,2(diarylphosphino)ethane, aryl = p-fluorobenzyl, benzyl,p-methyl-benzyl) have been prepared by protonating the precursor hydride complexes trans-[RuCl(H)(PP)2] using HBF4.Et20. These three dihydrogen complexes are quite stable and have been isolated in the solid state. The intact nature of the H-H bond in these derivatives has been established from the short spin-lattice relaxation times(T 1, ms) and the observation of substantial H, D couplings in the HD isotopomers. The H-H bond distances(d HH, å) increase from 0.97 to 1.01 å as the electron donor ability of the diphosphine ligand increases from the p-fluorobenzyl to the benzyl to the p-methylbenzyl moiety. These dihydrogen complexes constitute the initial stages of elongation of the H-H bond enroute to its cleavage along the reaction coordinate for the oxidative addition of H2 to a metal centre.