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.
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):
More information Group, Dimer ...
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,[2] 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[3] | 1,2,3,4-thiatriazol-5-thiolate | 1,2,3,4-thiatriazol-5-thiolato- | -yl 1,2,3,4-thiatriazol-5-thiolate | ~ CS2N3[4] | |
auride |
Au2
(Gold dimer) |
HAu
(Gold(I) hydride) |
auride |
aurido- |
|
~ Au |
−Au |
Close
Au− is considered to be a pseudohalogen ion due to its disproportionation reaction with alkali and the ability to form covalent bonds with hydrogen.[5]
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).
Margaret-Jane Crawford, et al. CS2N3, A Novel Pseudohalogen. J. Am. Chem. Soc. 2000, 122, 9052-9053
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.