A “vertex electron pair” scheme (VEPS) for describing the skeletal electron distribution in borane-type clusters

Abstract

Abstract A predominantly localized electron pair scheme is outlined for describing the electron distribution and bonding in closo borane anions B n H n 2− and related electron deficient deltahedral clusters, in which a skeletal electron pair is assigned to each vertex, one pair being regarded as delocalized just inside the roughly spherical surface on which the skeletal atoms lie. The scheme gives a clearer picture of the electron distribution than is conveyed by resonating 2- and 3-centre bonds in the polyhedron edges and faces, and allows the bond orders of the polyhedron edge links to be calculated readily. The consequence of formal removal of BH 2+ units from closo species B n H n 2− to generate nido species B n −1 H n −1 4− and arachno species B n −2 H n −2 6− is explored, and seen to allow rationalization of two features of such deltahedral-fragment clusters: (i) why a high-connectivity vertex is left vacant and (ii) why the frontier orbitals of such species concentrate electronic charge around their open faces. Moreover, in the case of D 4 `h B 4 H 4 6− (cf. C 4 H 4 2− ) and D 5 h B 5 H 5 6− (cf. C 5 H 5 − ), the approach leads directly to the familiar picture for aromatic ring systems in which the highest filled, doubly degenerate π-bonding molecular orbital concentrates electronic charge in rings above and below the polygon on which the skeletal nuclei lie. It also leads to the expectation that arachno clusters with non-adjacent vacant vertices will be more stable than those with adjacent vacant vertices.