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

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 sigma 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-infinity 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 Zn-2(BO)(2) cluster, the linear D-infinity h OBZnZnBO is the global minimum, in which the calculated Zn-Zn bond length of r(Zn-Zn) = 2.400 angstrom at the B3LYP level, which appears to be close to the latest recommended covalent radii (2.40 angstrom) of the proposed single bond covalent radii of the Zn-Zn bond. Chemical bonding analyses show that D-infinity h OBZnZnBO possesses a linear four-center two-electron (4c-2e) sigma bond. The sigma 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-infinity h HZnZnH and NCZnZnCN clusters also exhibit one linear 4c-2e sigma bond due to the secondary contribution from the H s and C sp components, respectively. The linear 4c-2e sigma bond greatly stabilizes the dizinc complexes. D-infinity 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-infinity v, (2)sigma(g)) -> OBZnZnBO (D-infinity h, (1)sigma(g)), is evaluated to be -58.75 kcal mol(-1) at the B3LYP level. Thus, D-infinity 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.