VSEPR square antiprism

Calculations, based on VSEPR theory so as to minimize repulsion between ligands in some of these afore-mentioned forms, have shown that repulsion energy coefficients for the square antiprism, dodecahedron, and cube are about the same but that the antiprism should be the most stable configuration for a MLg polyhedron (S). This principle has also been applied to K4[Mo(CN)g] 2H20, for which, in contrast... 

Rationalize the structure of [XeFg] (a square antiprism) in terms of VSEPR theory. 

In the field of large coordination numbers different geometries can be energetically very similar (77). This applies especially to coordination number 8. The atoms bonded to the metal can be arranged at the comers of a cube, as a square antiprism, or as a dodecahedron. The compound [Mo(CN)g] exemplifies this class and is dodecahedral, which the VSEPR would predict as very likely, but we are now stretching the theory to its very limit. This [MLg] structure appears to have 2 electrons not involved in dictating the geometry. 

For the most part, the structures of noble-gas compounds follow readily from VSEPR theory. In the gas phase, xenon hexafluoride assumes a nonrigid, slightly distorted octahedral structure, even though it has seven electron pairs around the central xenon atom. In the solid, it is characterized by square pyramidal XeF cations bridged by fluoride ions. XeF is a square antiprism, and XeOF4 is nearly a perfect square pyramid. 

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