Square antiprism modeling

Figure 9.28 (a) Points-on-a-sphere model of a square an-liprism. (b) The square antiprism geometry of the IFg2- ion. 

In the case of eight-coordination, the cube is fairly rare the compounds Na3[MFg] (M = Pa, U, Np) constitute the best-established examples. As for the trigonal prism versus the octahedron, interligand repulsion can be reduced by distortion of the cube to yield the square antiprism or dodecahedron (this is best demonstrated by playing with models). These two are about equally common. For example, in the case of Mo(CN)g-both square antiprismatic and dodecahedral coordination are found in the solid state in solution, the ion is stereochemically nonrigid (or fluxional) and appears to convert rapidly from one geometry to the other. 

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. ... 

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

Adding only one axial ligand to a square antiprism will result in a monocapped square antiprism (MSAP). The CN is 9 and the symmetry C4V For the most stable MSAP 0A 7O.r and 0b = 125.7° in the hard sphere model. Two 0 values are necessary to describe the structure. Because of the axial ligand, the (capped) square top face is larger than the square bottom face. Examples of rare-earth systems with a MSAP coordination polyhedron are the oxyhalogenides ROX (R = La, Y, Gd and X=C1, Br, I) (Holsa and Porcher 1981, 1982a). A square antiprism and a monocapped square antiprism are shown in fig. 15. 

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