Mesitylborane as a bis(σ-B-H) ligand:: An unprecedented bonding mode to a metal center

The new borane complex RuH2(eta(2):eta(2)-H(2)BMes)(PCy3)(2) (2) can be isolated in good yield by reaction of H(2)BMes with RuH2(eta(2)-H-2)(2)(PCy3)(2) (1) or alternatively, by reacting the chloro dihydrogen complex RuHCl(eta(2)-H-2)(PCy3)(2) (3) with lithium mesitylborohydride. 2 has been fully characterized by NMR and X-ray diffraction crystallography. A DFT/B3PW91 analysis shows that in 2, the H(2)BMes group is coordinated to the {RuH2(PCy3)(2)} fragment through two geminal sigma-B-H bonds. This coordination involves sigma-donation to the ruthenium and pi back-bonding from the ruthenium to the vacant p orbital of the boron. These two geminal sigma-B-H bonds are responsible for the short Ru-B distance (1.938 (4) angstrom by X-ray, 1.957 A by DFT).