Six-Vertex Hydrogen-Rich Cp2M2B4H8 Dimetallaboranes of the Second- and Third-Row Transition Metals: Effects of Skeletal Electron Count on Preferred Polyhedra

The complete series of hydrogen-rich six-vertex cyclopentadienyl dimetallaboranes Cp2M2B4H8 (Cp = η5-C5H5; M = Ir, Ru/Os, Re, Mo/W, and Ta), including the experimentally known Ir, Ru, and Re derivatives synthesized by Fehlner and co-workers, have now been examined by density functional theory. The nature of the central M2B4 polyhedra in the lowest energy Cp2M2B4H8 structures relates to the skeletal electron count as determined by the Wade–Mingos rules. Thus, the lowest energy Cp2Ir2B4H8 structures with 16 Wadean skeletal electrons have central pentagonal-pyramidal Ir2B4 units similar to that of the known pentagonal-pyramidal B6H10. The lowest energy Cp2M2B4H8 (M = Ru, Os) structures with 14 Wadean skeletal electrons have central capped-tetragonal-pyramidal rather than octahedral M2B4 units. However, isomeric Cp2M2B4H8 (M = Ru, Os) structures with central M2B4 octahedra are found at energies starting at ∼15 kcal/mol (M = Ru) and ∼10 kcal/mol (M = Os) above the capped-tetragonal-pyramidal global minima. The lowest energy electron poorer Cp2M2B4H8 structures (M = Re, Mo, W, Ta) have central M2B4 bicapped tetrahedra with the metal atoms at the degree 5 vertices. Higher energy Cp2Re2B4H8 structures include capped-tetragonal-pyramidal structures with surface ReRe double bonds and a pentagonal-pyramidal structure with a surface ReRe triple bond. The lowest energy Cp2M2B4H8 (M = Mo, W) structures appear to have surface MM double bonds and thus also the 12 skeletal electrons for their bicapped-tetrahedral structures. However, the lowest energy likewise bicapped-tetrahedral Cp2Ta2B4H8 structure is best interpreted in having CpTa units with 16-electron rather than 18-electron tantalum configurations and a surface Ta–Ta single bond.