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 Cp 2 M 2 B 5 H 9 (Cp = η 5 -C 5 H 5 ; 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 M 2 B 5 polyhedra depend on the skeletal electron count and, in most cases, can be related to the Wade–Mingos rules. Thus the six lowest-energy Cp 2 Ir 2 B 5 H 9 structures with 18 Wadean skeletal electrons are all based on the expected nido polyhedron obtained by removal of a degree 5 vertex from a D 2 d bisdisphenoid. Similar geometries are also found for most of the lowest energy Cp 2 M 2 B 5 H 9 structures (M = Ru, Os). The three lowest-energy Cp 2 Re 2 B 5 H 9 structures all have central Re 2 B 5 capped octahedra in accord with their 14 Wadean skeletal electrons. The low energy structures of the more hypoelectronic Cp 2 M 2 B 5 H 9 (M = Mo, W, Ta) systems have bicapped trigonal bipyramid geometries in which the two MB 2 faces of a central trigonal bipyramid are each capped by BH vertices. For Cp 2 M 2 B 5 H 9 (M = Mo, W) the experimentally realized symmetrical D 2 h 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.