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)

The intriguing question of whether it is possible to form a genuine M-0-M-0 single bond for the M-2 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 M-2(NHBMe)(2) (M = Zn, Cd, Hg; NHBMe = (HCNMe)(2)B) complexes in which the controlled donor-acceptor interaction leads to an M-0-M-0 single bond. In these complexes, M-2 in the (1)Sigma(g) ground state with the (n sigma(+)(g))(2)(n sigma(+)(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 (+,-) sigma-backdonation from the antibonding (n sigma(+)(u))(2) orbital of M-2 to the 2NHB(Me) ligands and a somewhat weaker (+,+) sigma-donation from the 2NHB(Me) ligands to the bonding (n + 1)sigma(+)(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 NHBMe ligand acts as a strong sigma-acceptor and a weaker sigma-donor. A comparison with the experimentally reported M-2(Ph-Dipp)(2) complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M-2(NHBMe)(2) complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M-2(NHBMe)(2) -> M + M(NHBMe)(2) and M-2(NHBMe)(2) -> M-2 + (NHBMe)(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 M-2(Ph-Dipp)(2) despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M-2(NHBMe)(2) complexes. Furthermore, the ligand exchange reaction considering M-2(Ph-Me)(2) as the starting material also turned out to be feasible. Therefore, the M-2(NHBMe)(2) complexes are the first cases that feature a neutral M-2 moiety with a single M-0-M-0 covalent bond, where M is a Group 12 metal.