Theoretical QTAIM, ELI-D, and Hirshfeld surface analysis of the Cu-(H)B interaction in [Cu2(bipy)2B10H10]

Interaction of [Cu2B10H10] with 2,2'-bipyridine (bipy) afforded a novel binuclear discrete complex of the [Cu2(bipy)2B10H10] composition. Two copper(I) atoms coordinate a bridge boron cage through an apical edge and a triangular BBB face situated at its opposite apical vertices to form four 3c2e (CuHB) and one 2c2e Cu-B bonds. The charge density model was obtained by density functional theory calculations of isolated molecule and crystal. The resultant densities were analyzed using the quantum theory of atoms in molecules (QTAIM) and electron localizability indicator (ELI-D). The geometry and the topological parameters of copper(I) coordination environment were found to be sensitive to crystal-field effect. An annulus of flat electron density ρ(r) and small ∇(2)ρ(r) is formed at dianion faces. As a result, some of the expected B-B, Cu-B, or Cu-H bond critical points are absent. The topological instability in the region of multicentered bonds is observed. The Cu-B bonding was found to be presumably electrostatic in nature, which could be the reason of topological isomerism for copper(I) decaborates. The results show that an unambiguous real-space criterion for multicentered bonding between transition metals and polyhedral boron anions is not yet given. The molecular graph for this class of compounds does not provide a definitive picture of the chemical boding and can be complemented with other descriptors, such as virial graphs and the ELI-D distribution.