Shameema Oottikkal, Jemmis Eluvathingal D
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore -12, India.
Inorg Chem. 2009 Aug 17;48(16):7818-27. doi: 10.1021/ic900741y.
The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) metallaboranes are explored using DFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C(6)H(6)MB(n-1)H(n-1) (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane B(n)H(n) suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry. This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams.
利用密度泛函理论（DFT）计算，探索了9、10、11和12顶点的闭式和超闭式（等闭式）金属硼烷的结构及电子关系。运用轨道兼容性的概念解释了过渡金属在稳定超闭式硼烷结构中的作用。将超闭式结构C(6)H(6)MB(n - 1)H(n - 1)（n = 9 - 12；M = Fe、Ru和Os）作为模型配合物。对不含金属的多面体硼烷B(n)H(n)的计算表明，n顶点的超闭式结构仅需要n个骨架电子对（SEP），但其结构会有一个或多个六度顶点，而具有n + 1个SEP的相应闭式结构只有四度和五度顶点。超闭式结构的这种高次顶点可被具有高度弥散轨道的过渡金属片段有效占据。计算进一步表明，具有更多弥散轨道的重过渡金属比轻过渡金属更倾向于形成超闭式几何结构。这与实验表征的含重过渡金属的超闭式结构更多这一事实相符。连接在金属原子上的外表面配体的大小在决定超闭式结构的稳定性方面也起作用。使用片段分子轨道方法分析了超闭式几何结构中硼烷与金属片段之间的相互作用。还根据相关图讨论了通过电子的添加和去除实现闭式和超闭式结构的相互转化。
