On the nature of bonding in a new boronyl species Zn2(BO)2: a linear four-center two-electron σ bond

Abstract

The Zn–Zn bond as one of the metal-to-metal bonds in clusters and molecules is of fundamental interest in many areas of natural science. Neutral boronyl can be viewed as a σ radical and is found in boronyl metal complexes. However, a complex with the Zn–Zn bond stabilized by boronyl ligands has not been found so far. Herein, we report on the computational design of the simplest case of such a system: linear D_∞h OBZnZnBO. The structural and electronic properties and chemical bonding on a series of zinc complexes Zn_x(BO)_y (x = 1,2; y = 1,2) with boronyl as ligands have been studied using quantum chemical calculations at the B3LYP and PBE0 levels, respectively. For the Zn₂(BO)₂ cluster, the linear D_∞h OBZnZnBO is the global minimum, in which the calculated Zn–Zn bond length of r_Zn–Zn = 2.400 Å at the B3LYP level, which appears to be close to the latest recommended covalent radii (2.40 Å) of the proposed single bond covalent radii of the Zn–Zn bond. Chemical bonding analyses show that D_∞h OBZnZnBO possesses a linear four-center two-electron (4c–2e) σ bond. The σ bond framework has a contribution of Zn orbitals 54% and B orbitals 44%, which involve Zn 4s 20% and 4p 34%, and B 2s 28% and 2p 16%, respectively. Furthermore, the D_∞h HZnZnH and NCZnZnCN clusters also exhibit one linear 4c–2e σ bond due to the secondary contribution from the H s and C sp components, respectively. The linear 4c–2e σ bond greatly stabilizes the dizinc complexes. D_∞h OBZnZnBO is thermochemically stable with respect to the possible formation channel at room temperature, whereas the formation energy of the exergonic channel, 2ZnBO (C_∞v, ²Σ_g) → OBZnZnBO (D_∞h, ¹Σ_g), is evaluated to be −58.75 kcal mol⁻¹ at the B3LYP level. Thus, D_∞h OBZnZnBO as the first observation of the Zn–Zn covalent bond in zinc complexes with boronyl as ligands may be synthesized in laboratories in the near future.