Unexpected Electronic Process of H2 Activation by a New Nickel Borane Complex: Comparison with the Usual Homolytic and Heterolytic Activations

H-H σ-bond activation promoted by Ni[MesB(o-Ph2PC6H4)2] (1(Mes)) was theoretically investigated with the density functional theory method. In 1(Mes), the nickel 3d, 4s, and 4p orbital populations are similar to those of the typical nickel(II) complex. First, one H2 molecule coordinates with the nickel center to form a dihydrogen complex, 2, which induces an increase in the nickel 3d and 4p orbital populations and thus a decrease in the nickel oxidation state. Then, the H-H σ-bond is cleaved under the unusual cooperation of the electron-rich nickel center and the electron-deficient borane ligand in a polarized manner, leading to an unprecedented trans-nickel(II) hydridoborohydrido complex, 3. In the transition state, charge transfer (CT) occuring from the H2 moiety to the 1(Mes) moiety (0.683 e) is much larger than the reverse CT (0.284 e). As a result, cleavage of the H-H σ-bond affords two positively charged hydrogen atoms. In this process, the boron atomic population and the nickel 4p orbital population increase, but the nickel 3d orbital population decreases. After cleavage of the H-H σ-bond, CT from the nickel 4p orbital to these positively charged hydrogen atoms occurs to afford 3, where the oxidation state of the nickel center increases to +2. These electronic processes are different from those of the usual homolytic and heterolytic H-H σ-bond activations. Regeneration of 1(Mes) and the role of the borane ligand in these reactions are also discussed in detail.