VSEPR 7-coordinate molecules

Coordination number refers to the number of electron pairs that surround a given atom we often refer to the atom of interest as the central atom even if this atom is not really located at the center of the molecule. If all of the electron pairs surrounding the central atom are shared with neighboring atoms, then the coordination geometry is the same as the molecular geometry. The application of VSEPR theory then reduces to the simple... 

Some of these trends are exemplified by the selection of molecules and complex ions in Table 1. They have been classified by (i) the total number of valence electrons (VE), and (ii) the steric number of the central atom (SN), which is calculated by adding the number of lone-pairs to the number of bonded atoms and used for interpreting molecular geometries in the VSEPR model (see Topic C2). The species listed in Table 1 illustrate the wide variety of isoelectronic relationships that exist between the compounds formed by elements in different groups and periods. Species with SN=4 are found throughout the p block, but ones with lower steric numbers and/or multiple bonding are common only in period 2. In analogous compounds with heavier elements the coordination and... 

The structure-determining role of the secondary bonds even in simple inorganic molecules and ions cannot be underestimated. Their formation results in increased coordination numbers and distortions of the coordination geometries. This matter was discussed in terms ofvalence bond and VSEPR theories, and a remarkable analysis of the coordination geometries o/Sb(III) halides influenced by secondary bonds could serve as a model for similar discussion of other main group elements. 

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