The first nonmetal-centered binuclear sandwich-like complexes based on the tetraatomic species E42- (E = N, P, As, Sb, Bi) and boron atoms

The first nonmetal-centered binuclear sandwich-like complexes, B-2(eta(4)-E-4)(2) (E = CMe, N, P, As, Sb, Bi; Me = CH3), have been investigated by density functional theory (DFT) to explore whether the nonmetal element can be at the center to form stable sandwich-like complexes. The stable conformer for each species is the D-4d staggered one, in which there exists strong interaction between the two boron atoms. Natural bonding orbital (NBO) analysis indicates that the boron-boron bonds are all sigma single bonds, which are predicted to be derived mostly from the s and p(z) orbitals of the boron atoms. The boron-boron bond dissociation energies are higher than that of the Zn-Zn bond in the synthesized complex Cp*ZnZnCp* (Cp* = C5Me5; Me = CH3) suggesting that these nonmetal-centered sandwich-like complexes may be synthesized in future experiments. According to the energy decomposition analysis (EDA), the boron-boron bond is much weaker than the boron-ligand bond and the ability of the E-4(2-) ligands to stabilize the boron-boron bonds is in the order of N-4(2-) > P-4(2-) > As-4(2-) > Sb-4(2-) > Bi-4(2-). Nucleus-independent chemical shift (NICS) values reveal that E-4(2-) (E = CMe, N, P, As, Sb) rings of B-2(eta(4)-E-4)(2) exhibit characteristics of pi aromaticity at 1.0 angstrom above the ring center, whereas Bi-4(2-) rings of B-2(eta(4)-Bi-4)(2) possess antiaromaticity. In addition, the differences in the bonding nature between the nonmetal-centered and metal-centered binuclear sandwich-like complexes were also investigated. In these nonmetal-centered complexes B-2(eta(4)-E-4)(2), the interactions between the boron atoms and the E(4)(2-)ligands are more than half covalent, while in the metal-centered binuclear sandwich-like complexes, the metal-ligand interactions are mainly ionic.