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.本研究通过密度泛函理论（density functional theory），对完整系列的富氢六顶点环戊二烯基二金属硼烷Cp₂M₂B₄H₈（Cp = η⁵-C₅H₅；M分别为铱(Ir)、钌(Ru)/锇(Os)、铼(Re)、钼(Mo)/钨(W)以及钽(Ta)）开展了系统理论研究，其中涵盖了费尔纳（Fehlner）及其合作者通过实验合成的Ir、Ru及Re衍生物。该类化合物最低能量构型的中心M₂B₄多面体结构性质，与基于韦德-明戈斯（Wade–Mingos）规则确定的骨架电子数密切相关。具体而言，具有16个韦德型骨架电子的最低能量Cp₂Ir₂B₄H₈构型，其中心Ir₂B₄单元呈五角锥型，与已报道的B₆H₁₀五角锥型结构高度相似。对于拥有14个韦德型骨架电子的最低能量Cp₂M₂B₄H₈（M = Ru、Os）构型，其中心M₂B₄单元为帽式四方锥型结构，而非八面体型。不过，带有中心八面体型M₂B₄单元的Cp₂M₂B₄H₈（M = Ru、Os）异构体，其能量分别比帽式四方锥型全局最低构型高出约15 kcal/mol（M = Ru）与10 kcal/mol（M = Os）。骨架电子数更少的最低能量Cp₂M₂B₄H₈构型（M = Re、Mo、W、Ta），其中心M₂B₄单元为双帽四面体结构，且金属原子位于五配位顶点处。能量更高的Cp₂Re₂B₄H₈构型包括带有表面Re=Re双键的帽式四方锥型结构，以及带有表面Re≡Re三键的五角锥型结构。最低能量的Cp₂M₂B₄H₈（M = Mo、W）构型似乎带有表面M=M双键，因此其双帽四面体结构同样对应12个骨架电子。然而，同样为双帽四面体结构的最低能量Cp₂Ta₂B₄H₈构型，最佳诠释为其CpTa单元为16电子而非18电子的钽配位构型，且表面存在Ta-Ta单键。