Geometry and chemical bonding in polyhedral boranes, metallaboranes, and dimetallaboranes: From closo to isocloso to oblatocloso polyhedra

The geometry and chemical bonding in the closo   metal-free boranes BnHn2- and the isoelectronic carboranes CBn-1Hn- and C2Bn−2Hn with 2n + 2 skeletal electrons are based on the most spherical deltahedra with a preference for degree 5 vertices, particularly for the boron atoms. Such deltahedral boranes can be considered to be three-dimensional aromatic systems, as indicated by strongly diatropic nucleus independent chemical shift values for BnHn2- (n = 6, 8, 9, 12). Metallaborane structures, particularly those with 9–11 vertices and only 2n rather than 2n + 2 apparent skeletal electrons, are often based on isocloso deltahedra with the metal atom at a degree 6 vertex. Dimetallaborane structures, particularly the rhenium derivatives Cp2Re2Bn−2Hn−2 (8 ⩽ n ⩽ 12), are based on highly non-spherical and very oblate deltahedra with the metal atoms typically at degree 6 or 7 vertices, which are the lowest curvature sites of the deltahedra. A viable model for the skeletal bonding in such dimetallaboranes can be developed if each of the two metal vertices is assumed to contribute five internal orbitals to the skeletal bonding. This leads to 2n + 4 skeletal electrons, which are partitioned into n surface bonds and a formal metal–metal double bond inside the oblate deltahedron.

The closo → isocloso → oblatocloso deltahedra for metal-free boranes, metallaboranes, and dimetallaboranes, respectively, all have n formal surface bonds. The closo deltahedra supplement their surface bonding with an n-center two-electron core bond, which is absent in the corresponding isocloso metallaborane deltahedron. The oblatocloso dimetallaboranes contain a formal metal–metal double bond inside the deltahedron in addition to the surface bonding.Figure optionsDownload as PowerPoint slide