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Polyhedral skeletal electron pair theoryIn chemistry the polyhedral skeletal electron pair theory provides electron counting rules used to predict the structure of electron deficient clusters. They were originally formulated by K. Wade and further developed by Mingos and others, and are sometimes known as Wade's Rules or Wade / Mingos rules. The rules were originally developed to aid the prediction of the structures of borane and carborane cluster compounds and are based on a molecular orbital treatment of the bonding. Additional recommended knowledge
The rules for boranes and carboranesThe skeletal electron counts for the four types of deltahedral cluster are:-
The skeletal electron counts are determined by totalling the following:-
Total electron count formulation for boranes and carboranesThere is an alternative and more straightforward version of the rules that uses a total valence electron count. For boranes and carboranes the total valence electrons are counted plus any charge electrons,the rules are simply:-
Theoretical basis for the rules for boranesA brief insight into the theory behind the rules through a consideration of the anionic closo cluster B6H62-. The boron atoms sit at the vertices of an octahedron, each using one sp hybrid, that points radially out of the polyhedron, to form a σ-bond to a hydrogen atom. Each boron atom therefore has 3 orbitals remaining to contribute to framework molecular orbitals i.e.:
The 18 framework molecular orbitals, (MOs), derived from the 18 boron atomic orbitals are :-
The total skeletal bonding orbitals is therefore 7 i.e (n+1). Isolobal vertex unitsProvided a vertex unit is isolobal with BH then it can, in principle at least, be substituted for a BH unit. The CH+ unit is isolobal, hence the reason why the rules are applicable to carboranes.
Transition metal clustersAs for boranes and carboranes there is an alternative and more straightforward version of the rules that uses a total valence electron count. The total valence electrons are counted plus any charge electrons. The rules are simply:-
General References
Categories: Chemical bonding | Inorganic chemistry | Organometallic chemistry |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Polyhedral_skeletal_electron_pair_theory". A list of authors is available in Wikipedia. |