Suffixes onium

Some variants are also allowed for mononuclear cations of Groups 15, 16 and 17. These are based on the usage of substitutive nomenclature, where the formal addition of a hydron to a parent hydride to give a cation is represented by the suffix -onium. 

At the time this was written by Ingold (1953, vide supra), the suffix onium was used for this species. However, it has subsequently been (largely) agreed to reserve onium for positively charged species with filled valence shells (e.g., ammonium [NH4 ], oxonium [R3O ]). Current use has the words carbonium ion for the species RsC7 replaced by the single word carbocation. ... 

A carbocationic species in which there is at least one pentavalent carbon atom (e.g., CHs ). 2. Traditional name for chemical species that are now referred to as carbenium ions. Considerable confusion exists in the literature with this term for carbocations. The -onium suffix usually refers to a higher covalency when compared to the neutral atom thus, CH5+ would be a true carbonium ion (in terms of the first definition). Additional ambiguity results when the term ethyl carbonium ion is used to describe both CH3CH2 and to R—CH2CH2. For these reasons, the terms carbocation or carbenium ion are now preferred. 

Many organic chemists, and indeed the previous versions of this book, use the term carbonium ion for species of this kind. However, there is well-established usage of the -onium suffix, for ammonium, oxonium, chloronium, and so on, to denote positively charged atoms with filled valence shells. In the interest of greater uniformity of nomenclature we shall use carbocation for carbon positive ions that have unfilled valence shells (6 electrons). 

Suffix to indicate addition of hydrons to a parent hydride or other parent structure (see anium , enium , inium , onium , ynium ), accompanied by multiplying prefixes and locants as appropriate, e.g. hydrazinium, H2NNH3+ hydrazine- 1,2-diium, +H3NNH3+. 

Subtraction of two and three H-atoms is expressed by -ylidene, -ylidyne, with the exception of methylene ( CH2), silylene ( SiH2) etc., and aminylene For cations comprising core elements of groups 15-17 the traditional -onium suffixes and -onio- prefixes are still prefered. 

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