The new borane complex RuH
2(
2:
2-H
2BMes)(PCy
3)
2 (
2) can be isolated in good yield by reaction of H
2BMes with RuH
2(
2-H
2)
2(PCy
3)
2 (
1) or alternatively, by reacting the chloro dihydrogen complex RuHCl(
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
2BMes group is coordinated to the {RuH
2(PCy
3)
2} fragment through two geminal
-B-H bonds. This coordination involves
-donation to the ruthenium and
back-bonding from the ruthenium to the vacant p orbital of the boron. These two geminal
-B-H bonds are responsible for the short Ru-B distance (1.938 (4) Å by X-ray, 1.957 Å by DFT).