Hypercoordinate Nonclassical Carbonium Ions

The second class of carbocations contains one or more hypercoordinate carbon atoms. These hypercarbons are coordinated to five or more atoms within reasonable bonding distance. Hypercoordinate or nonclassical carbocations cannot be described by using 2c-2e single bonds alone, but necessitate the involvement of three- (or multi-) center, two-electron bonds, e.g., 3c-2e bonds. Each hypercarbon in the cation is always associated with eight electrons, although the ion, overall, is electron deficient. The methonium ion (Cd I5 ) may be considered the parent of the hypercoordinate carbocations (Fig. 5.1). 

The two classes of carbocations are not mutually exclusive but represent the limits of a spectrum of carbocations with various types and degrees of charge delocalization. The interaction of neighboring groups with the vacant p-orbital of a carbenium ion center can contribute to the ions stability. This may involve the interaction of unshared electron pairs (n-donors), bent 

Roberts was the first to use the term nonclassical ion when he proposed the tricyclobutonium ion structure for the cyclopropylmethyl cation. Winstein referred to the nonclassical structure of norbomyl, cholesteryl, and 

3-phenyl-2-butyl cations. P. D. Bartlett s definition was, however, widely used for long for nonclassical ions An ion is nonclassical if its ground state has delocalized bonding a electrons.  

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

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