PSEP model

While the capping and fusion rules significantly increase the power of the PSEP model for clusters whose atoms have relatively high metal-metal connectivity, the newest extentions apply the model to lower connectivity clusters. For transition metals, 15n electrons (where n is the number of vertices) are expected for clusters whose metal atoms are connected to three other metal atoms (that is, three-connected clusters). " Examples include tetrahedra, cubes, and cuneanes. In the four-connected clusters, such as octahedra, square antiprisms, and cuboctahedra, the electron count can range from 14n to 14n -I- 4, depending on the nature of the polyhedron. ... 

GTD model requires an increase in the number of skeletal orbitals in the internal atoms. According to the PSEP model, which was originally employed in boron cluster chemistry and only later applied to metal clusters, these orbitals are considered to be nonbonding. Thus, the GTD model implicitly emphasizes the importance of metal d orbitals in cluster bonding. In so doing, it emphasizes that capped and fused structures are highly unusual in boron hydride clusters due to the boron atoms possessing only four s and p valence AOs which thereby restrict the number of internal cluster orbitals to three. 

Polyhedral Skeletal Electron Pair (PSEP) Model... 

Transition metal analogues of closo boranes are known. They are characterized by a number of valence electrons equal to 14n+2 as a consequence of the filling of the additional five d orbitals. Also, tranation metal polyhedra exhibiting nido and arachno structures (with electron counts of 14n+4 and 14n+6 respectively) have been identified. [227] Thus, the PSEP model represents a very ample, yet powerful method to rationalize or predict the structures of duster compounds... 

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