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 (dichlorane)	Hydrogen chloride (chlorane)	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 (dioxidane)	water (oxidane)	hydroxide	hydroxido- hydroxy-	-ol	~ OH	−O−H
sulfanyl	hydrogen disulfide (disulfane)	hydrogen sulfide (sulfane)	hydrosulfide	sulfanido- thiolato-	-thiol -yl mercaptane	~ SH	−S−H
selanyl	hydrogen diselenide (diselane)	hydrogen selenide (selane)	hydroselenide	selanido- selenolato-	-selenol	~ SeH	−Se−H
tellanyl	hydrogen ditelluride (ditellane)	hydrogen telluride (tellane)	hydrotelluride	tellanido- tellurolato-	-tellurol	~ TeH	−Te−H
cyanate	dicyanodioxidane	cyanic acid	cyanate	cyanato-	-yl cyanate	~ OCN	−O−C≡N
isocyanate	isocyanogen	isocyanic acid	isocyanate	isocyanato-	-yl isocyanate	~ NCO	−N=C=O
fulminate	fulminogen	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	isothiocyanogen	isothiocyanic acid	isothiocyanate	isothiocyanato-	-yl isothiocyanate	~ NCS	−N=C=S
selenocyanate, selenorhodanide	selenocyanogen	selenocyanic acid	selenocyanate	selenocyanato-	-yl selenocyanate	~ SeCN	−Se−C≡N
tellurocyanate, tellurorhodanide	tellurocyanogen	tellurocyanic acid	tellurocyanate	tellurocyanato-	-yl tellurocyanate	~ TeCN	−Te−C≡N
azide	azidogen	hydrazoic acid	azide	azido-	-yl azide	~ N3	−N−−N+≡N ↕ −N=N+=N−
nitric oxide	dinitrogen dioxide	nitroxyl	nitroxide	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(−C≡N)3
1,2,3,4-thiatriazol-5-thiolate	bis(1,2,3,4-thiatriazol-5-yl)disulfane	1,2,3,4-thiatriazol-5-thiol	1,2,3,4-thiatriazol-5-thiolate	1,2,3,4-thiatriazol-5-thiolato-	-yl 1,2,3,4-thiatriazol-5-thiolate	~ CS2N3
auride	Au2 (Gold dimer)	HAu (Gold(I) hydride)	auride	aurido-		~ Au	−Au

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 pseudohalogen compounds (Ps−Ps, where Ps is a pseudohalogen) include cyanogen (CN)₂, thiocyanogen (SCN)₂ and hydrogen peroxide H₂O₂. Another complex symmetrical pseudohalogen compound 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 pseudohalogen compounds (pseudohalogen halides Ps−X, where Ps is a pseudohalogen and X is a halogen, or interpseudohalogens Ps¹−Ps², where Ps¹ and Ps² are two different pseudohalogens), analogous to the binary interhalogen compounds, are cyanogen halides like cyanogen chloride (Cl−CN), cyanogen bromide (Br−CN), nitryl fluoride (F−NO₂), nitrosyl chloride (Cl−NO) and chlorine azide (Cl−N₃), as well as interpseudohalogens like dinitrogen trioxide (O=N−NO₂), nitric acid (HO−NO₂) and cyanogen azide (N₃−CN).

Not all combinations of interpseudohalogens and pseudohalogen halides are known to be stable (e.g. sulfanol HS−OH).

Pseudohalides

Pseudohalides form univalent anions which form binary acids with hydrogen and form insoluble salts with silver such as silver cyanide (AgCN), silver cyanate (AgOCN), silver fulminate (AgCNO), silver thiocyanate (AgSCN) and silver azide (AgN₃).

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

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 hydrogen chloride HCl to hydrogen tetracarbonylcobaltate HCo(CO)₄), and they can react with metals M to form compounds like MX (compare sodium chloride NaCl to sodium azide NaN₃).

Nanoclusters of aluminium (often referred to as superatoms) are sometimes considered to be pseudohalides since they, too, behave chemically as halide ions, forming Al₁₃I−2 (analogous to triiodide I−3) and similar compounds. This is due to the effects of metallic bonding on small scales.