Pseudohalogen

Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.

Common pseudohalogens and their nomenclature

Many pseudohalogens are known by specialized common names according to where they occur in a compound. Well-known ones include (the true halogen chlorine is listed for comparison):

Group	Dimer	Hydrogen compound	Pseudohalide	Ligand name	In organic compounds	Formula	Structural formula
True halogens
chloro	chlorine	Hydrogen chloride	chloride	chlorido- chloro-	-yl chloride	~ Cl	−Cl
Pseudohalogens
cyano	cyanogen	hydrogen cyanide, prussic acid, formonitrile	cyanide	cyanido- cyano-	-nitrile -yl cyanide	~ CN	−C≡N
cyapho	cyaphogen	phosphaethyne	cyaphide	cyaphido- cyapho-	-yl cyaphide	~ CP	−C≡P
isocyano	isocyanogen	hydrogen isocyanide, isohydrocyanic acid	isocyanide	isocyanido- isocyano-	-isonitrile -yl isocyanide	~ NC	−N+ ≡C−
hydroxyl	hydrogen peroxide	water	hydroxide	hydroxido- hydroxy-	-ol	~ OH	−O−H
sulfanyl	hydrogen disulfide	hydrogen sulfide	hydrosulfide	sulfanido- thiolato-	-thiol -yl mercaptane	~ SH	−S−H
cyanate		cyanic acid	cyanate	cyanato-	-yl cyanate	~ OCN	−O−C≡N
isocyanate		isocyanic acid	isocyanate	isocyanato-	-yl isocyanate	~ NCO	−N=C=O
fulminate		fulminic acid	fulminate	fulminato-	-nitrile oxide -yl fulminate	~ CNO	−C≡N+ –O−
thiocyanate, rhodanide	thiocyanogen	thiocyanic acid	thiocyanate	thiocyanato-	-yl thiocyanate	~ SCN	−S−C≡N
isothiocyanate		isothiocyanic acid	isothiocyanate	isothiocyanato-	-yl isothiocyanate	~ NCS	−N=C=S
selenocyanate, selenorhodanide	selenocyanogen	selenocyanic acid	selenocyanate			~ SeCN	−Se−C≡N
tellurocyanate, tellurorhodanide	tellurocyanogen	tellurocyanic acid	tellurocyanate			~ TeCN	−Te−C≡N
azide	hexazine	hydrazoic acid	azide	azido-	-yl azide	~ N3	−N− −N+ ≡N ↕ −N=N+ =N−
nitric oxide	dinitrogen dioxide	nitroxyl		nitrosyl	nitroso-	~ NO	•N=O
nitrogen dioxide	dinitrogen tetroxide			nitryl	nitro-	~ NO2	−NO2
cobalt carbonyl	dicobalt octacarbonyl	cobalt tetracarbonyl hydride	tetracarbonylcobaltate			~ Co(CO)4	−Co(C≡O)4
trinitromethanide	hexanitroethane	nitroform, trinitromethane	trinitromethanide	trinitromethanido-	-yl trinitromethanide	~ C(NO2)3	−C(NO2)3
tricyanomethanide	hexacyanoethane	cyanoform, tricyanomethane	tricyanomethanide	tricyanomethanido-	-yl tricyanomethanide	~ C(CN)3	−C(CN)3
						~ CS2N3

Au⁻ is considered to be a pseudohalogen ion due to its disproportionation reaction with alkali and the ability to form covalent bonds with hydrogen.

Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogens (Ps–Ps) include cyanogen (CN)₂, thiocyanogen (SCN)₂, hydrogen peroxide H₂O₂. Another complex symmetrical pseudohalogen is dicobalt octacarbonyl, Co₂(CO)₈. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co(CO)₄.

Examples of non-symmetrical pseudohalogens (Ps–X), analogous to the binary interhalogen compounds, are cyanogen halides like ClCN or BrCN; nitryl fluoride, and other compounds. Sometimes nitrosyl chloride NOCl also is considered as pseudohalogen.

Not all combinations are known to be stable.

Pseudohalides

Pseudohalides are the univalent anions (or functional groups) which form hydracids with hydrogen and form insoluble salts with silver such as cyanide, cyanates, fulminate, thiocyanate and azide.

A common complex pseudohalide is a tetracarbonylcobaltate [Co(CO)
₄]⁻
. The acid HCo(CO)₄ is in fact quite a strong acid, though its low solubility renders it not as strong as the true hydrohalic acids.

The behavior and chemical properties of the above pseudohalides are identical to that of the true halide ions. The presence of the internal multiple bonds does not appear to affect their chemical behavior. For example, they can form strong acids of the type HX (compare HCl to HCo(CO)₄), and they can react with metals to form compounds like MX (compare NaCl to NaN₃).

Nanoclusters of aluminium (often referred to as superatoms) are sometimes considered to be pseudohalides since they, too, behave chemically as halide ions, forming Al₁₃I₂⁻ (analogous to I₃⁻) and similar compounds. This is due to the effects of metallic bonding on small scales.