Unprecedented Bonding Situation in Viable E2 (NHBMe )2 (E=Be, Mg; NHBMe =(HCNMe )2 B) Complexes: Neutral E2 Forms a Single E-E Covalent Bond.

Abstract

Is it possible to facilitate the formation of a genuine Be-Be or Mg-Mg single bond for the E2 species while it is in its neutral state? So far, (NHCR )Be-Be(NHCR ) (R=H, Me, Ph) have been reported where Be2 is in 1 Δg excited state imposing a formal Be-Be bond order of two. Herein, we present the formation of a single E-E (E=Be, Mg) covalent bond in E2 (NHBMe )2 (E=Be, Mg; NHBMe =(HCNMe )2 B) complexes where E2 is in 3 ∑u + excited state having (nσg + )2 (nσu + )1 ((n+1)σg + )1 (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron-shared bonding with two NHBMe radicals. The effects of bonding with nσu + and (n+1)σg + orbitals will cancel each other, providing the former E-E bond order as one. Be2 (NHBMe )2 complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg2 (NHBMe )2 → Mg + Mg(NHBMe )2 and Mg2 (NHBMe )2 → Mg2 + (NHBMe )2 , are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol-1 , respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E2 moiety with a single E-E covalent bond.