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The stability of η2-H2 borane complexes - a theoretical investigation.

Non-metallic η(2)-H2 complexes are extremely rare; moreover in the case of boranes (with the exception of the BH5 molecule) the existence of such structures was only indicated by computational studies. In a recent paper we have demonstrated that external electron donor groups can stabilize the η(2)-H2 complexes, similar to the backdonation in the case of transition metals. In this paper we present evidence of a new stabilizing effect: electron donation from the B-R bonds to the H2 σ* orbital. The stability and electronic structure of several mono-, di-, and trisubstituted borane-H2 complexes were investigated by ab initio calculations. SiR3 groups were found to facilitate the σ(B-R)→σ*(H-H) interaction, increasing the stability of the η(2) complexes. Furthermore in the case of tris(trimethyl)silylborane, the exceptional stability of a novel neutral pentavalent borane structure is shown.