Unprecedented Bonding Situation in Viable E₂(NHB^Me)₂(E=Be, Mg; NHB^Me=(HCN^Me)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, (NHC^R )Be-Be(NHC^R ) (R=H, Me, Ph) have been reported where Be₂ is in ¹ Δ_g 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 (NHB^Me )₂ (E=Be, Mg; NHB^Me =(HCN^Me )₂ B) complexes where E₂ is in ³ ∑_u ⁺ excited state having (nσ_g ⁺ )² (nσ_u ⁺ )¹ ((n+1)σ_g ⁺ )¹ (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron-shared bonding with two NHB^Me radicals. The effects of bonding with nσ_u ⁺ and (n+1)σ_g ⁺ orbitals will cancel each other, providing the former E-E bond order as one. Be₂ (NHB^Me )₂ complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg₂ (NHB^Me )₂ → Mg + Mg(NHB^Me )₂ and Mg₂ (NHB^Me )₂ → Mg₂ + (NHB^Me )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.