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

The new borane complex RuH 2 (η 2:η 2 -H 2 BMes)(PCy 3 ) 2 ( 2 ) can be isolated in good yield by reaction of H 2 BMes with RuH 2 (η 2 -H 2 ) 2 (PCy 3 ) 2 ( 1 ) or alternatively, by reacting the chloro dihydrogen complex RuHCl(η 2 -H 2 )(PCy 3 ) 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 {RuH 2 (PCy 3 ) 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).