Configurations for Atoms with Unshared Electron Pairs

CONFIGURATIONS FOR ATOMS WITH UNSHARED ELECTRON PAIRS 

A considerable amount of information about the relative orientations of bonds formed by an atom that also possesses one or more unshared electron pairs has been gathered. 

In a few cases an unshared electron pair seems to have no effect on bond directions. This is observed98 for Se(IV) in the octahedral complex ion [SeBre]— and for Sb(III) in [SbBr ].  

Usually, however, an unshared pair seems to occupy one of the comers of a coordination polyhedron and to replace the shared pair of a bond. The molecules NH , PC1, etc. have pyramidal configurations that might be described as involving bonds directed toward three corners of a tetrahedron with the fourth corner occupied by the unshared pair, and a similar description can be given for H 0, (CH ) S, and related molecules. 

The extension of this postulate to atoms with five bonds and one unshared pair suggests that the bonds should be directed toward the five corners of a square pyramid, which with the unshared pair would form an octahedron. BrF has been shown94 to have this configuration. The bromine atom lies about 0.15 A below the base of the pyra- 

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