Filling the void: controlled donor–acceptor interaction facilitates the formation of an M–M single bond in the zero oxidation state of M (M = Zn, Cd, Hg)

Abstract

The intriguing question of whether it is possible to form a genuine M⁰–M⁰ single bond for the M₂ species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively M^I–M^I. Herein, we present viable M₂(NHB^Me)₂ (M = Zn, Cd, Hg; NHB^Me = (HCN^Me)₂B) complexes in which the controlled donor–acceptor interaction leads to an M⁰–M⁰ single bond. In these complexes, M₂ in the ¹∑_g ground state with the (nσ_g⁺)²(nσ_u⁺)² (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor–acceptor bonding with singlet 2NHB^Me ligands where a combined effect of dominant (+,−) σ-backdonation from the antibonding (nσ_u⁺)² orbital of M₂ to the 2NHB^Me ligands and a somewhat weaker (+,+) σ-donation from the 2NHB^Me ligands to the bonding (n + 1)σ_g⁺ orbital leads to the unorthodox bonding situation of forming an M–M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHB^Me ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M₂(Ph^Dipp)₂ complexes reveals the uniqueness of the NHB^Me ligand in exhibiting such a bonding scenario. The M₂(NHB^Me)₂ complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M₂(NHB^Me)₂ → M + M(NHB^Me)₂ and M₂(NHB^Me)₂ → M₂ + (NHB^Me)₂. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5–39.5 kcal mol⁻¹. The experimental characterization of M₂(Ph^Dipp)₂ despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M₂(NHB^Me)₂ complexes. Furthermore, the ligand exchange reaction considering M₂(Ph^Me)₂ as the starting material also turned out to be feasible. Therefore, the M₂(NHB^Me)₂ complexes are the first cases that feature a neutral M₂ moiety with a single M⁰–M⁰ covalent bond, where M is a Group 12 metal.