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

Three new monocationic molecular hydrogen complexes of ruthenium of the type trans- $[RuC1(\eta2-H_2)(PP)_2][BF_4]$ (PP = bis-1,2(diarylphosphino)ethane, aryl = p-fluorobenzyl, benzyl, p-methylbenzyl) have been prepared by protonating the precursor hydride complexes trans-$[RuC1(H)(PP)_2]$ using $HBF_4.Et_2O$. 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 $(d_{HH}$, \AA) increase from 0.97 to 1.01 \AA 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 $H_2$ to a metal centre.