The Wade–Mingos rules in seven-vertex dimetallaborane chemistry: Hydrogen-rich Cp2M2B5H9 systems of the second and third row transition metals

Abstract

The complete series of hydrogen-rich seven-vertex cyclopentadienyl dimetallaboranes Cp2M2B5H9 (Cp = η5-C5H5; M = Ir, Ru/Os, Re, Mo/W, Ta), including the experimentally known Mo and W derivatives, have been examined by density functional theory. The central M2B5 polyhedra depend on the skeletal electron count and, in most cases, can be related to the Wade–Mingos rules. Thus the six lowest-energy Cp2Ir2B5H9 structures with 18 Wadean skeletal electrons are all based on the expected nido polyhedron obtained by removal of a degree 5 vertex from a D2d bisdisphenoid. Similar geometries are also found for most of the lowest energy Cp2M2B5H9 structures (M = Ru, Os). The three lowest-energy Cp2Re2B5H9 structures all have central Re2B5 capped octahedra in accord with their 14 Wadean skeletal electrons. The low energy structures of the more hypoelectronic Cp2M2B5H9 (M = Mo, W, Ta) systems have bicapped trigonal bipyramid geometries in which the two MB2 faces of a central trigonal bipyramid are each capped by BH vertices. For Cp2M2B5H9 (M = Mo, W) the experimentally realized symmetrical D2h bicapped trigonal bipyramid structure of this type is obviously very favorable since it lies more than 23 kcal/mol in energy below the next lowest energy structure.