Ions and Radicals

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Adducts, involving a dipolar bond with electron donation from and/or to [pi]-orbitals. GTPOC (pi adduct).

ampholytes: See zwitterionic compounds.

anhydro bases:

Compounds resulting from internal acid-base neutralization (with loss of water) in iminium hydroxides containing an acidic site conjugated with the iminium function. Cf. pseudo bases.


Originally, the compound betaine, (CH3)3N+CH2C(=O)O- N,N,N-trimethylammonioacetate, and similar zwitterionic compounds derived from other amino acids. NOC Rule C-816.1, footnote. By extension, neutral molecules having charge-separated forms with an onium atom which bears no hydrogen atoms and that is not adjacent to the anionic atom. Betaines cannot be represented without formal charges. See also dipolar compounds, mesoionic compounds, ylides, zwitterionic compounds. E.g. (CH3)3P+CH2S(=O)2O-, (Ph)3P+CH2CH2O-.


Although this term has been recommended in the past for diradicals , specialists working in the field prefer the latter term. GTPOC (biradical)

Bunte salts:

Salts (usually sodium salts) of S-alkylthiosulfuric acid, of general structure RSS(=O)2O- M+. Use of this term is discouraged.


Anions containing an even number of electrons in which the excess negative charge is formally located on one or more carbon atoms. E.g. H3C- methyl anion or methanide, H3CC-(=O) acetyl anion or 1-oxoethanide, H3CC-HCH3 isopropyl anion or propan-2-ide, cyclopenta-2,4-dienyl anion or cyclopenta-2,4-dien-1-ide. See also acyl intermediates, enolates. GTPOC (carbanions).


This prefix is now rejected. It has been used in the past, especially in cyclic systems, to name carbenes. E.g. 'carbenacycloheptane', now cycloheptylidene.


The electrically neutral species H2C: and its derivatives, in which the carbon is covalently bonded to two univalent groups of any kind or a divalent group and bears two nonbonding electrons, which may be spin-paired (singlet state) or spin-non-paired (triplet state). In systematic name formation, carbene is the name of the parent hydride :CH2 ; hence, the name dichlorocarbene for :CCl2. However, names for acyclic and cyclic hydrocarbons containing one or more divalent carbon atoms are derived from the name of the corresponding all-[lambda]4-hydrocarbon using the suffix -ylidene. RNRI Rule RC-81.1.3. NOC Rule C-81.1. E.g. CH 2=CHCH: prop-2-en-1-ylidene; cyclohexylidene, H2C=C: ethenylidene.

Subclasses of carbenes include acyl carbenes RC(=O)C:R, imidoyl carbenes, RC(=NR)C:R, and vinyl carbenes.

carbene analogues:

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The electrically neutral mononuclear hydrides of group 14 having two non-bonding electrons, the electrically neutral mononuclear hydrides of group 15 having four non-bonding electrons, and also the compound HB:. The names of a number of these, formed according to RNRI Rules RC-, RC-, are shown below. These names are entries in this Glossary.

R2Si: silylenesRP:phosphanylidenes
R2Ge:germylidenes RAs:arsanylidenes
R2Sn: stannylidenesRSb: stibanylidenes
R2Pb: plumbylidenes

Note that if R = H, these compounds are parent hydrides; derivatives formed by substitution are named accordingly. However, if the substituent's first atom, bearing the free valence, is of the same element as the atom of the carbene analogue, other parent compounds may be required (see carbenes), E.g. HN: nitrene; CH3N: methylnitrene; H2NN: diazanylidene (synonym isodiazene, not aminonitrene).

carbene radical anions:

Species R2C[3dot]-, having three non-bonding electrons, formally derived by addition of an electron to a carbene.

carbene radical cations:

Species R2C.+, having one non-bonding electron, formally derived by subtraction of an electron from a carbene . E.g. H2C.+ methyliumyl.

carbenium ions:

A structure, real or hypothetical, representing a carbocation which contains at least one carbon atom having only six valence electrons, i.e. 'formally trivalent'. GTPOC (carbenium ions). Cf. carbonium ions, vinylic cations. E.g. CH3C+HCH3 propan-2-ylium, CH3OCH2+ methoxymethylium.


Complexed carbene-like entities that display the reactivity characteristics of carbenes, either directly or by acting as sources of carbenes.

carbinyl cations: An obsolete term, derived from carbinol, once used for carbenium ions.


Cations containing an even number of electrons in which the positive charge is formally located on one or more carbon atoms. See also alkanium ions , carbenium ions, vinyl cations . GTPOC (carbocations). RNRI Rule RC-, footnote ***.

carbonium ions: An older term applied usually to carbenium ions but also to alkanium ions and other carbocationic species. The term is then ambiguous and not recommended. Cf. carbocations.

carbonyl ylides:

1,3-Dipolar compounds having the structure R2C=O+-C-R2 [2head arrow] R2C+-OC-R2. See also oxonium ylides (2).


The neutral species HC[3dot] and its derivatives formed by substitution in which a univalent carbon atom is covalently bonded to one group and also bears three nonbonding electrons. (This term carries no implication about spin-pairing.) RNRI Rule RC-

carbynium ions:

The cationic species H2C.+ or substitution derivatives thereof, formally derived by adding a hydron to a carbyne or subtracting an electron from a carbene.

charge-transfer complexes:

Electron-donor--electron-acceptor complexes, characterised by electronic transition(s) to an excited state in which there is a partial transfer of electronic charge from the donor to the acceptor moiety. GTPOC (charge-transfer complexes).

[pi]-complexes: See [pi]-adducts.


Molecular entities bearing two negative charges, which may be located on a single atom or on different atoms or may be delocalized.

dicarbenium ions:

Species carrying two positive charges, formally located on tervalent carbon atoms.

E.g. +CH2-+CH2 ethane-1,2-diyl dication, +CH2CH2-+CH2 propane-1,3-bis(ylium).

dipolar compounds:

Electrically neutral molecules carrying a positive and a negative charge in one of their major canonical descriptions. In most dipolar compounds the charges are delocalized; however the term is also applied to species where this is not the case. 1,2-Dipolar compounds have the opposite charges on adjacent atoms. The term 1,3-dipolar compounds is used for those in which a significant canonical resonance form can be represented by a separation of charge over three atoms (in connection with 1,3-dipolar cycloadditions). Subclasses of 1,3-dipolar compounds include:

(i) Allyl type

X=Y+-Z- [2head arrow] -X-Y+=Z [2head arrow] +X-Y-Z- [2head arrow] -X-Y-Z+ (X, Z = C, N, or O; Y = N or O) See azo imides, azomethine imides, azomethine ylides, azoxy compounds, carbonyl imides, carbonyl oxides, carbonyl ylides, nitrones, nitro compounds.

(ii) Propargyl type

X[triple bond]N+-Z- [2head arrow] -X=N+=Z [2head arrow] -X=N-Z+ [2head arrow] X-N=Z (X = C or O, Z = C, N, or O) See nitrile imides, nitrile oxides, nitrile ylides, nitrilium betaines, azides, diazo compounds.

(iii) Carbene type

:X-C=Z [2head arrow] +X=C-Z- (X = C or N; Z = C, N, or O) See acyl carbenes, imidoyl carbenes, vinyl carbenes.

See betaines. R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, A. Padwa, Ed.,Vol. 1, Wiley, New York, 1984, p. 3.

E.g. RN--N+[triple bond]N [2head arrow] RN=N+=N- [2head arrow] RN--N=N+ ; RC[triple bond]N+-O- [2head arrow] RC-=N+=O [2head arrow] RC+=N-O-

dipolar ions: See zwitterionic compounds.


Molecular species having two unpaired electrons, in which at least two different electronic states with different multiplicities [electron-paired (singlet state) or electron-unpaired (triplet state)] can be identified. E.g. H2C.-CH 2C.H2 propane-1,3-diyl (trimethylene). See biradicals. GTPOC (biradicals).

inner salts: See zwitterionic compounds.

ion pairs:

Pairs of oppositely charged ions held together by coulombic attraction without formation of a covalent bond. Experimentally, an ion pair behaves as one unit in determining conductivity, kinetic behaviour, osmotic properties, etc.

ion radicals: See radical ions.

keto carbenes:

The term has imprecisely been used to designate carboxylic acyl carbenes. Keto carbenes are carbenes bearing an oxo function at an unspecified site.


Radical anions (or the corresponding salts) derived from ketones by addition of an electron: R2C.-O- [2head arrow] R2C--O.. NOC Rule C-84.4. Note: Ketyls produce two types of conjugate acids: R2C.-OH and R2CH-O.. The former are [alpha]-hydroxyalkyl radicals and the latter are alkoxyl radicals, but they have also been called ketyls in photochemistry.

Meisenheimer complexes/compounds/adducts:

Originally, this term was applied to the somewhat stable, detectable delocalized cyclohexadienyl anions (or salts) formed by addition of alkoxide ions to the ortho or para positions of an ortho- or para-substituted nitrobenzene, particularly in the case of nitroaryl ethers. It has since been generalized to include other anionic addends (and even uncharged nucleophiles), other activating groups besides nitro (or even none at all), any substituent (or none) at the site of addition, and heterocyclic analogues. GTPOC (Meisenheimer adduct).

mesoionic compounds:

Dipolar five- (possibly six-) membered heterocyclic compounds in which both the negative and the positive charge are delocalized, for which a totally covalent structure cannot be written, and which cannot be represented satisfactorily by any one polar structure. 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 . W. D. Ollis, S. P. Stanforth and C. A. Ramsden, Tetrahedron 41, 2239-2329 (1985).


Mesoionic compounds having a 1,3-oxazole skeleton bearing an oxygen atom attached to the 5-position with the following delocalised structure:

onium compounds:*

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1. Cations (with their counterions) derived by addition of a hydron to a mononuclear parent hydride of the nitrogen, chalcogen and halogen families. NOC Rules C-82.1, D-5.33.

H4N+ ammoniumH3O+oxoniumH2F+fluoronium
H4P+ phosphoniumH3S+ sulfoniumH2 Cl+chloronium
H4As+ arsoniumH3Se+ selenoniumH2Br+bromonium
H4Sb+ stiboniumH3Te+ telluroniumH2I+iodonium

2. 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- E.g. Cl2F+ dichlorofluoronium, (CH3)2S+H dimethylsulfonium (a secondary sulfonium ion), ClCH3)3P+ chlorotrimethylphosphonium, (CH3CH2)4N+ tetraethylammonium (a quaternary ammonium ion). See also quaternary ammonium compounds, phosphonium compounds, arsonium compounds, stibonium compounds, oxonium ions, sulfonium compounds, halonium ions.

3. Derivatives formed by substitution of the above parent ions by groups having two or three free valencies on the same atom. Such derivatives are, where possible, designated by a specific class name. E.g. RC[triple bond]O+ hydrocarbylidyne oxonium ions, R2C=N+H2 X - iminium compounds, RC[triple bond]NH+ nitrilium ions.

oxenium ions:

An unnecessary and erroneous term for oxylium ions, RO + (:O:: is monooxygen, not "oxene").

[alpha]-oxo carbenes: Synonymous with acyl carbenes . See also keto carbenes.

oxonium ions:

The parent ion H3O+ and substitution derivatives thereof. See under onium compounds.

oxonium ylides:

1. Compounds having the structure R2O+-C-R2. See ylides.

2. A class of 1,3-dipolar compounds of general structure R2C=O+-Y-, comprising carbonyl imides, carbonyl oxides and carbonyl ylides.

oxylium ions:

Species of the form RO+. RNRI Rule RC- E.g. CH 3O+ methoxylium, HO+ hydroxylium.

pi-adducts: See [pi]-adducts under 'a'.

pi-complexes: See [pi]-adducts under 'a'.


1. Molecular entities possessing an unpaired electron, such as .CH3, .SnH3, .Cl. (In these formulae the dot, symbolizing the unpaired electron, should be placed so as to indicate the atom of highest spin density, if this is possible). Paramagnetic metal ions are not normally regarded as radicals. Depending on the nature of the core atom that possesses an unpaired electron, the radicals can be described as carbon-, oxygen-, nitrogen-, metal-centered radicals. Subclasses are e.g. acyl, acyloxyl, alkyl, aminoxyl, aminyl, aryl, diazenyl, hydrocarbylsulfanyl radicals, iminoxyl, iminyl, silyl. See also diradicals.

2. In the past, the term 'radical' was used to designate a substituent group bound to a molecular entity, as opposed to 'free radical,' which nowadays is called simply 'radical'. The bound entities may be called groups or substituents, but should no longer be called radicals.

radical anions: See radical ions.

radical cations: See radical ions.

radical ions:

Species with an odd number of valence electrons and a positive or negative charge, called, respectively, radical cations, R.+, and radical anions, R.-. Commonly, but not necessarily, the odd electron and the charge are associated with the same atom. See also ketyls.

Radical cations e.g. R2O[3dot]+ R3N.+ [CH4].+ ;

Radical anions e.g. RN.-O- [2 headed arrow] RN--O[dot] ; RN(O.)(O-)

Note: In mass spectroscopic usage the symbol for the charge precedes the dot representing the unpaired electron. GTPOC (radical ion). In the electrochemical tradition dot and sign are arranged vertically e.g. R[dot over plus] or R[plus over dot].

radical pairs:

Two radicals in close proximity in the solid or gas phase or, when in solution, within a common solvent cage.


Radical anions having the structure -O-Z-O. where Z is an ortho- or para-arylene group or analogous heteroarylene group; they are formally generated by the addition of an electron to a quinone.


The diradical CH2=C(CH 2.)2, 2-methylenepropane-1,3-diyl, for which no Kekulé structure can be written, and its hydrocarbyl derivatives. J. A. Berson, Acc. Chem. Res. 11, 446-453 (1978).

vinyl carbenes:

Carbenes having a vinylic group on a carbenic carbon atom: R2C=CRCR: [2headed arrow] R2C+-CR=C-R: . See under 1,3-dipolar compounds. E.g. H2 C=CHCH: prop-2-en-1-ylidene.

vinylic cations:

Carbocations having the structure R2 C=C+-R.


Carbenes in which the carbenic carbon atom has a double bond to another carbon atom: R2C=C: . E.g. H2C=C: ethenylidene.


Compounds in which an anionic site Y- (originally on carbon, but now including other atoms) is attached directly to a heteroatom X+ (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. NOC Rule D-5.42. They are thus 1,2-dipolar species of the type RmX+-Y-Rn. If X is a saturated atom of an element from the first row of the periodic system, the ylide is commonly represented by a charge-separated form; if X is a second, third, etc. row element uncharged canonical forms are available: RmX=YRn. If X is an unsaturated atom, doubly bonded to another first row element Z, the negative charge on Y may be stabilised by [pi]-conjugation, Z=X+-Y-R n [2headed arrow] Z--X+=YRn. Such ylides belong to the class 1,3-dipolar compounds. However, 1,3-dipolar compounds with only sextet-containing canonical forms (e.g. vinylcarbenes) are not ylides. See also betaines, dipolar compounds. E.g. Ph3P+-C-H2 [2headed arrow] Ph3P=CH2 (often called a Wittig reagent), (CH3)3N+-C-H2, RC[triple bond]N+N--R, (CH3)2S=CHPh [2headed arrow] (CH3)2S+-C-HPh.

Note that ylide is a complete word, not to be confused with the suffix -ylide, used for some radical anions.

Subclasses of ylides. Ylides RmX+-C-R2 having the negative charge on carbon are classified by citing the name of the element X before the word ylide. E.g. nitrogen ylide, phosphorus ylide, oxygen ylide, sulfur ylide. A further specification may be achieved by citing the class name of RmX before the word ylide. Thus nitrogen ylides include amine ylides, R3N+-C-R2, azomethine ylides, R2C=N+R-C-R2, nitrile ylides, RC[triple bond]N+-C -R2. Some authors, who wish to express the positive charge on X, prefer e.g. ammonium ylides over amine ylides; such usage varies according to the heteroatom X and to national custom. The ylides RmX+-Y- [2headed arrow] RmX=Y (Y = O, S, Se, Te, NR) are usually named by citing the name of RmX followed by the additive nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively). NOC Rules C-0.3, D-5.41. E.g. amine imides; use of the less systematic synonyms amine imines and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. nitrones, Wittig reagents (synonymous with phosphonium ylides)

zwitterionic compounds/zwitterions:

Neutral compounds having formal unit electrical charges of opposite sign. Some chemists restrict the term to compounds with the charges on non-adjacent atoms. Sometimes referred to as inner salts, dipolar ions (a misnomer). See betaines, dipolar compounds, ylides. E.g. H3N+CH2C(=O) O ammonioacetate (glycine), (CH3)3N+-O- trimethylamine oxide.

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