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.[1] 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	Acid	Pseudohalide	Ligand name	Organic suffix	Formula	Structural formula
chloro	chlorine (for example)	hydrochloric		chlorido- chloro-	-yl chloride	~ Cl	~ Cl
cyano	cyanogen	hydrocyanic prussic	cyanide	cyanido- cyano-	-nitrile -yl cyanide	~ CN	~ C≡N
cyapho	cyaphogen	hydrocyaphic	cyaphide	cyaphido- cyapho-	-yl cyaphide	~ CP	~ C≡P
isocyano	isocyano-formonitrile	isohydrocyanic	isocyanide	isocyanido- isocyano-	-isonitrile -yl isocyanide	~ NC	~ N≡C
hydroxyl	hydrogen peroxide	water	hydroxide bioxide	hydroxido- hydroxy-	-ol	~ OH	~ O−H
sulfhydryl	hydrogen disulfide	hydrogen sulfide	hydrosulfide bisulfide	sulfanido- thiolato-	-thiol	~ SH	~ S−H
cyanate	—	cyanic	cyanate	cyanato-	-yl cyanate	~ OCN	~ O−C≡N
isocyanate	—	isocyanic	isocyanate	isocyanato-	-yl isocyanate	~ NCO	~ N≡C−O
fulminate	—	fulminic	fulminate	fulminato-	-yl fulminate	~ CNO	~ C≡N−O
thiocyanate, rhodanide	thiocyanogen	thiocyanic	thiocyanate	thiocyanato-	-yl thiocyanate, -yl isothiocyanate	~ SCN	~ S−C≡N
isothiocyanate		isothiocyanic	isothiocyanate	isothiocyanato-	-yl isothiocyanate	~ NCS	~ N=C=S
hypothiocyanite		hypothiocyanous	hypothiocyanite	thiocyanito-	-yl hypothiocyanite	~ OSCN
selenocyanate, selenorhodanide	selenocyanogen	selenocyanic	selenocyanate			~ SeCN	~ Se−C≡N
tellurocyanate, tellurorhodanide[2]	tellurocyanogen	tellurocyanic	tellurocyanate			~ TeCN	~ Te−C≡N
azide	hexazine	hydrazoic	azide	azido-	-yl azide	~ N3	~ N^− −N^+ ≡N ↕ ~ N=N^+ =N^−
nitrogen dioxide	dinitrogen tetroxide	nitrous	nitrite	nitro-	-yl nitrite	~ NO2
cobalt carbonyl	dicobalt octacarbonyl	cobalt tetracarbonyl hydride	tetracarbonylcobaltate(1−)	?	?	~ Co(CO)4	~ Co(−C≡O)4
trinitromethanide	hexanitroethane	trinitromethane	trinitromethanide	trinitromethanido- (IUPAC)	-yl trinitromethanide	C(NO2)3^−	C(NO2)3^−
tricyanomethanide	hexacyanoethane	tricyanomethane(cyanoform)	tricyanomethanide	tricyanomethanido- (IUPAC)	-yl tricyanomethanide	C(CN)3^−	C(CN)3^−
trifluoromethanesulfonate	—	triflic acid	trifluoromethanesulfonate, triflate	triflate (-yl) trilfate	-yl triflate	~ O3SCF3−	~ OTf

This list is incomplete; you can help by expanding it.

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), analogous to the binary interhalogen compounds, 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 univalent anions (or functional groups) which form hydracids with hydrogen and form insoluble salts with Ag(Silver) such as cyanides, cyanates, isocyanates, rhodanides (i.e. thiocyanates and isothiocyanates), selenocyanogens, tellurorhodanides and azides.

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 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.

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "pseudohalogens". doi:10.1351/goldbook.P04930
2. http://www.chemspider.com/Chemical-Structure.21865273.html