Dihydrogen Adduct (Co–H2) Complexes Displaying H‐Atom and Hydride Transfer

The prototypical reactivity profiles of transition metal dihydrogen complexes (M‐H2) are well‐characterized with respect to oxidative addition (to afford dihydrides, M(H)2) and as acids, heterolytically delivering H+ to a base and H− to the metal. In the course of this study we explored plausible alternative pathways for H2 activation, namely direct activation through H‐atom or hydride transfer from the σ‐H2 adducts. To this end, we describe herein the reactivity of an isostructural pair of a neutral S=1/2 and an anionic S=0 Co‐H2 adduct, both supported by a trisphosphine borane ligand (P3B). The thermally stable metalloradical, (P3B)Co(H2), serves as a competent precursor for hydrogen atom transfer to tBu3ArO⋅. What is more, its anionic derivative, the dihydrogen complex [(P3B)Co(H2)]1−, is a competent precursor for hydride transfer to BEt3, establishing its remarkable hydricity. The latter finding is essentially without precedent among the vast number of M‐H2 complexes known. Isostructural Co–H2 adducts are stabilized as highly unusual neutral S=1/2 and anionic S=0 species. The atypical reactivity of these complexes is described, specifically their propensity for accessing H‐atom and hydride transfer.