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.<ref>Template:GoldBookRef</ref> 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	Template:Chem2
Pseudohalogens
hydrido	hydrogen	dihydrogen	hydride	hydrido-	-ane -ene -yne others	~ H	Template:Chem2
cyano	cyanogen	hydrogen cyanide, prussic acid, formonitrile	cyanide	cyanido- cyano-	-nitrile -yl cyanide	~ CN	Template:Chem2
cyapho	cyaphogen	phosphaethyne	cyaphide	cyaphido- cyapho-	-yl cyaphide	~ CP	Template:Chem2
isocyano	isocyanogen	hydrogen isocyanide, isohydrocyanic acid	isocyanide	isocyanido- isocyano-	-isonitrile -yl isocyanide	~ NC	Template:Chem2
hydroxyl	hydrogen peroxide (dioxidane)	water (oxidane)	hydroxide	hydroxido- hydroxy-	-ol	~ OH	Template:Chem2
sulfanyl	hydrogen disulfide (disulfane)	hydrogen sulfide (sulfane)	hydrosulfide	sulfanido- thiolato-	-thiol -yl mercaptane	~ SH	Template:Chem2
selanyl	hydrogen diselenide (diselane)	hydrogen selenide (selane)	hydroselenide	selanido- selenolato-	-selenol	~ SeH	Template:Chem2
tellanyl	hydrogen ditelluride (ditellane)	hydrogen telluride (tellane)	hydrotelluride	tellanido- tellurolato-	-tellurol	~ TeH	Template:Chem2
cyanate	dicyanodioxidane	cyanic acid	cyanate	cyanato-	-yl cyanate	~ OCN	Template:Chem2
isocyanate	isocyanogen	isocyanic acid	isocyanate	isocyanato-	-yl isocyanate	~ NCO	Template:Chem2
fulminate	fulminogen	fulminic acid	fulminate	fulminato-	-nitrile oxide -yl fulminate	~ CNO	Template:Chem2
thiocyanate, rhodanide	thiocyanogen	thiocyanic acid	thiocyanate	thiocyanato-	-yl thiocyanate	~ SCN	Template:Chem2
isothiocyanate	isothiocyanogen	isothiocyanic acid	isothiocyanate	isothiocyanato-	-yl isothiocyanate	~ NCS	Template:Chem2
selenocyanate, selenorhodanide	selenocyanogen	selenocyanic acid	selenocyanate	selenocyanato-	-yl selenocyanate	~ SeCN	Template:Chem2
citation	CitationClass=web }}</ref> tellurorhodanide || tellurocyanogen|| tellurocyanic acid|| tellurocyanate|| tellurocyanato- || -yl tellurocyanate|| ~ TeCN || Template:Chem2
azide	azidogen	hydrazoic acid	azide	azido-	-yl azide	~ Template:Chem2	Template:Chem2 ↕ Template:Chem2
nitric oxide	dinitrogen dioxide	nitroxyl	nitroxide	nitrosyl	nitroso-	~ NO	Template:Chem2
nitrogen dioxide	dinitrogen tetroxide			nitryl	nitro-	~ Template:Chem2	Template:Chem2
cobalt carbonyl	dicobalt octacarbonyl	cobalt tetracarbonyl hydride	tetracarbonylcobaltate			~ Template:Chem2	Template:Chem2
trinitromethanide	hexanitroethane	nitroform, trinitromethane	trinitromethanide	trinitromethanido-	-yl trinitromethanide	~ Template:Chem2	Template:Chem2
tricyanomethanide	hexacyanoethane	cyanoform, tricyanomethane	tricyanomethanide	tricyanomethanido-	-yl tricyanomethanide	~ Template:Chem2	Template:Chem2
1,2,3,4-thiatriazol-5-thiolate	bis(1,2,3,4-thiatriazol-5-yl)disulfane	1,2,3,4-thiatriazol-5-thiol<ref name=pubchem>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref> || 1,2,3,4-thiatriazol-5-thiolate || 1,2,3,4-thiatriazol-5-thiolato- || -yl 1,2,3,4-thiatriazol-5-thiolate || ~ Template:Chem2<ref>Margaret-Jane Crawford, et al. CS2N3, A Novel Pseudohalogen. J. Am. Chem. Soc. 2000, 122, 9052-9053</ref> || File:1,2,3,4-Thiatriazol-5-ylthio group.png
auride	Au2 (Gold dimer)	HAu (Gold(I) hydride)	auride	aurido-	Organogold chemistry	~ Au	Template:Chem2

Template:Chem2 is considered to be a pseudohalogen ion due to its disproportionation reaction with alkali and the ability to form covalent bonds with hydrogen.<ref>Template:Cite journal</ref>

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Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogen compounds (Template:Chem2, where Ps is a pseudohalogen) include cyanogen Template:Chem2, thiocyanogen Template:Chem2 and hydrogen peroxide Template:Chem2. Another complex symmetrical pseudohalogen compound is dicobalt octacarbonyl, Template:Chem2. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Template:Chem2.

Examples of non-symmetrical pseudohalogen compounds (pseudohalogen halides Template:Chem2, where Ps is a pseudohalogen and X is a halogen, or interpseudohalogens Template:Chem2, where Template:Chem2 are two different pseudohalogens), analogous to the binary interhalogen compounds, are cyanogen halides like cyanogen chloride (Template:Chem2), cyanogen bromide (Template:Chem2), nitryl fluoride (Template:Chem2), nitrosyl chloride (Template:Chem2) and chlorine azide (Template:Chem2), as well as interpseudohalogens like dinitrogen trioxide (Template:Chem2), nitric acid (Template:Chem2) and cyanogen azide (Template:Chem2).

Not all combinations of interpseudohalogens and pseudohalogen halides are known to be stable (e.g. sulfanol Template:Chem2).

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 (Template:Chem2).

A common complex pseudohalide is a tetracarbonylcobaltate Template:Chem2. The acid cobalt tetracarbonyl hydride Template:Chem2 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 Template:Chem2), and they can react with metals M to form compounds like MX (compare sodium chloride NaCl to sodium azide Template:Chem2).

Nanoclusters of aluminium (often referred to as superatoms) are sometimes considered to be pseudohalides since they, too, behave chemically as halide ions, forming [[Superatom#Aluminum clusters|Template:Chem2 (analogous to triiodide Template:Chem2) and similar compounds. This is due to the effects of metallic bonding on small scales.