Pseudohalogen

The 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	Molecule	Acid	Pseudohalide	Ligand name	Organic suffix	Formula	Structural formula
chlorine	chlorine	hydrochloric	chloride	chlorido- (IUPAC), chloro-	-yl chloride	~ Cl	~ Cl
cyano	cyanogen	hydrocyanic, prussic	cyanide	cyanido- (IUPAC), cyano-	-nitrile	~ CN	~ C≡N
isocyanide	isocyanogen	isohydrocyanic, isoprussic	isocyanide		-isonitrile	~ NC	~ N≡C
cyanate	—	isocyanic	cyanate	cyanato-	-yl cyanate	~ OCN	~ O−C≡N
isocyanate	—	isocyanic	isocyanate	isocyanato-	-yl isocyanate	~ NCO	~ N≡C-O
thiocyanate, rhodanide	thiocyanogen	thiocyanic	thiocyanate	thiocyanato-	-yl thiocyanate, -yl isothiocyanate	~ SCN	~ S−C≡N
isothiocyanate			isothiocyanate	isothiocyanato-	-yl isothiocyanate	~ NCS	~ N=C=S
hypothiocyanite		hypothiocyanous	hypothiocyanite	thiocyanito-	-yl hypothiocyanite	~ OSCN
selenocyanate, selenorhodanide	selenocyanogen	selenocyanic	selenocyanate			~ SeCN	~ Se−C≡N
azide	—	hydrazoic	azide	azido-	-yl azide	~ N3	~ N− −N+ ≡N ↕ ~ N=N+ =N−
cobalt carbonyl	dicobalt octacarbonyl	cobalt tetracarbonyl hydride	tetracarbonylcobaltate(1−)	?	?	~ Co(CO)4	~ Co(−C≡O)4
trinitromethanide	trinitromethane	trinitromethanic	trinitromethanide	trinitromethanido- (IUPAC), trinitromethanido-	-yl trinitromethanide	C(NO2)3^−	C(NO2)3^−
tricyanomethanide	tricyanomethane	tricyanomethanic	tricyanomethanide	tricyanomethanido- (IUPAC), tricyanomethanido-	-yl tricyanomethanide	C(CN)3^−	C(CN)3^−
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Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogens (Ps–Ps) include cyanogen (CN)₂, thiocyanogen (SCN)₂, selenorhodane (SeCN)₂, azidodithiocarbonate (N₃CS₂)₂. 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) are cyanogen halides (ICN, ClCN, BrCN), and other compounds. Sometimes nitrosyl chloride NOCl also is considered as pseudohalogen.

Not all combinations are known to be stable.

Pseudohalides

Pseudohalides are the anions (or functional groups) of corresponding pseudohalogen groups such as cyanides, cyanates, isocyanates, rhodanides (i.e. thiocyanates and isothiocyanates), selenocyanogens, tellurorhodanides and azides.

A common complex pseudohalide is 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 double bonds or triple bonds do 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.