Mesitylborane as a Bis(σ-B−H) Ligand:  An Unprecedented Bonding Mode to a Metal Center

The new borane complex RuH₂(η²:η²-H₂BMes)(PCy₃)₂ (2) can be isolated in good yield by reaction of H₂BMes with RuH₂(η²-H₂)₂(PCy₃)₂ (1) or alternatively, by reacting the chloro dihydrogen complex RuHCl(η²-H₂)(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 H₂BMes group is coordinated to the {RuH₂(PCy₃)₂} fragment through two geminal σ-B−H bonds. This coordination involves σ-donation to the ruthenium and π back-bonding from the ruthenium to the vacant p orbital of the boron. These two geminal σ-B−H bonds are responsible for the short Ru−B distance (1.938 (4) Å by X-ray, 1.957 Å by DFT).