The term has imprecisely been used to designate carboxylic acyl carbenes . Keto carbenes are carbenes bearing an oxo function at an unspecified site. NOC Rule C-84.4. The former are -hydroxyalkyl radicals and the latter are alkoxyl radicals, but they have also been called ketyls in photochemistry. The formal positive charge is associated with the ring atoms, and the formal negative charge is associated with ring atoms or an exocyclic nitrogen or chalcogen atom.
Mesoionic compounds are a subclass of betaines .
See also munchnones , sydnones . Derivatives formed by substitution of the above parent ions by univalent groups. The number of substituted hydrogen atoms is, especially in the case of hydrocarbyl substituents, indicated by the adjectives primary, secondary, tertiary or quaternary. RNRI Rule RC-82.1.1.1. See also quaternary ammonium compounds , phosphonium compounds , arsonium compounds , stibonium compounds , oxonium ions , sulfonium compounds , halonium ions .