Structures derived from a trigonal bipyramid

There are 10 electrons around the central atom in XeF2, so the linear structure is derived from a trigonal bipyramid with the unshared pairs of electrons in equatorial positions (D,.fl symmetry). 

Agreement is poor for small clusters, but this does not present a fair test—the smallest structure type considered is that derived from a trigonal bipyramid having an S value of 6. However, the predictive power for clusters of four to six atoms, for which the rules perform best, is only moderate. Even in that region, correspondence with the actual... 

In P4 structure, each P atom contributes three valence electrons to the skeletal valence electron count. Since there are four P atoms, there are 12 valence skeletal electrons or six skeletal electron pairs. So P4 would be n + 2 cluster (n = 4) making the structure nido type. Formally, its structure then should be derived from a trigonal bipyramid. This is indeed a reasonable solution because removal of one vertex from a trigonal bipyramid leaves tetrahedral structure. 

The structure of SF4, 15.26, is derived from a trigonal bipyramid and can be rationalized in terms of VSEPR theory. The S—F and S—F q bond distances are quite different (Table 15.5). Oxidation by O2 in the absence of a... 

Some XY3-type halides take the unusual T-shaped structure of C2V symmetry shown below. This geometry is derived from a trigonal-bipyramidal structure in which two equatorial positions are occupied by two lone-pair electrons. Typical examples are CIF3 and BrF3. With the equatorial Y atom represented as Y, the following assignments have been made for these molecules [725] v(XY ), Ai, 754 and 672 v(XY), Bi 683.2 and 597 v(XY), Ai, 523 and 547 5, Ai 328 and 235 5, 431 and 347 tt, 2,... 

It has long been known that molecules such as SF4, SeF4, and TeF4 assume a distorted tetrahedral structure (C21 ) derived from a trigonal-bipyramidal geometry with a lone-pair of elections occupying an equatorial position ... 

As for tetranuclear clusters, pentanuclear transition metal clusters are only capable of partially encapsulating a main-group heteroatom. The c/o5o-pentanuclear cluster, the trigonal bipyramid, has an interstitial cavity of similar dimension to that of the tetrahedron, and must therefore open-up if it is to accommodate a semi-interstitial atom. The most commonly observed M5E clusters (E = main-group atom) adopt square based pyramidal structures, which can be derived from the trigonal bipyramid by M-M bond cleavage or can, alternatively, be viewed as /i/Jo-octahedral species (Scheme 2). 

The preferred molecular shape will depend on the number of spatial pairs that have spread apart around the atom on which they reside (i) two pairs yield linear structures, (ri) three pairs yield planar structures with bond angles of 120°, (iii) four pairs lead to structures derived from a tetrahedron with bond angles of 109°, (iv) five pairs lead to structures derived from trigonal bipyramidal structures, with bond angles of 120° and 90°, and (v) six pairs lead to structures derived from an octahedron, with bond angles of 90°. 

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