Inorganic and Physical Chemistry Dept. , Indian Institute of Science , Bangalore 560012 , Karnataka , India.
Inorg Chem. 2019 Mar 18;58(6):3627-3634. doi: 10.1021/acs.inorgchem.8b02986. Epub 2019 Feb 27.
The removal of H atoms from polyhedral boranes results in the formation of dangling radial orbitals with one electron each. If there is a requirement of electrons for skeletal bonding to meet the Wade's rule, these are provided from the exohedral orbitals. Additional electrons occupy a linear combination of the dangling orbitals. Stabilization of these molecular orbitals depends on their overlap. The lateral (sideways) overlap of dangling orbitals decreases with the decreasing cluster size from 12 to 5 boron atoms as the orbitals become more and more splayed out. Thus, as the number of dangling orbitals increases, the destabilization of their combinations increases at a higher rate for smaller polyhedral boranes, leading to flat structures with the removal of a fewer number of hydrogens. Though exohedral orbitals form better overlap in larger polyhedral clusters, the increase of electrons with the removal of H atoms results in occupancy of antibonding skeletal orbitals (beyond Wade's rules) and leads to flat structures. The reverse happens when hydrogens are added to a flat cluster. Substitution of BH by Si does not change structural patterns.
多面体硼烷脱去 H 原子会形成每个原子带有一个孤电子的悬空径向轨道。如果骨架键合需要电子来满足 Wade 规则，这些电子就来自于外部轨道。额外的电子占据悬空轨道的线性组合。这些分子轨道的稳定性取决于它们的重叠。随着硼原子从 12 个减少到 5 个，悬空轨道的横向（侧向）重叠减少，因为轨道变得更加张开。因此，随着悬空轨道数量的增加，较小多面体硼烷的组合失稳率以更高的速率增加，导致在去除较少数量的氢原子后形成平面结构。尽管外部轨道在较大的多面体簇中形成更好的重叠，但随着 H 原子的去除，电子的增加导致占据反键骨架轨道（超出 Wade 规则），并导致平面结构。当向平面簇中添加氢原子时，情况正好相反。用 Si 取代 BH 不会改变结构模式。
