Hyperconjugation isovalent

This latter example is sometimes called an example of isovalent hyperconjugation (the contributing structure containing the same number of two-electron bonds as the normal Lewis formula). 

On the theoretical level there is no serious question of the possible importance of hyperconjugation in systems for which more than one classical structure can be written. This is what Mulliken (74a) calls isovalent hyperconjugation, and in the valence bond picture can be of three basic types ... 

XXII show that as the electron demand of the ligand on mercury increases the s character in the C-H and C-C bonds increases in agreement with Bent s isovalent model(18). In addition,/wr-< 7t hyperconjugation between the methylene C-H bonds and the mercury atom is revealed by plotting versus... 

The moment we admit that the carbonyl carbon atom is electron deficient, i.e., the molecule is stabilized by structures in which there is a vacant 7r-orbital on the carbonyl carbon, we cto no loi r logically exclude hyperconjugation with structures that are isovalent with this... 

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