Unprecedented Bonding Situation in Viable E2(NHBMe)2(E=Be, Mg; NHBMe=(HCNMe)2B) Complexes: Neutral E2Forms 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 1Δg 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)2B) 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.