The stability of η2-H2 borane complexes – a theoretical investigation
Non-metallic η2-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 η2-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 η2 complexes. Furthermore in the case of tris(trimethyl)silylborane, the exceptional stability of a novel neutral pentavalent borane structure is shown.