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Pseudohalogen

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Compounds with similar properties to polyatomic halogens
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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
hydrido hydrogen dihydrogen hydride hydrido- -ane
-ene
-yne
others		 ~ H −H
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- Organogold chemistry ~ 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.

This list is incomplete; you can help by adding missing items. (September 2014)

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 Ps−Ps, where Ps and Ps 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.

References

  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "pseudohalogens". doi:10.1351/goldbook.P04930
  2. "Tellurocyanate | CNTe | ChemSpider". Archived from the original on 2021-11-21.
  3. "5-Mercapto-1,2,3,4-thiatriazole".
  4. Margaret-Jane Crawford, et al. CS2N3, A Novel Pseudohalogen. J. Am. Chem. Soc. 2000, 122, 9052-9053
  5. Mudring, Anjy-Verena; Jansen, Martin (2000-11-28). "ChemInform Abstract: Base-Induced Disproportionation of Elemental Gold". ChemInform. 31 (48): no. doi:10.1002/chin.200048020.
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