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, 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 :| Group | Dimer | Acid | Pseudohalide | Ligand name | Organic suffix | Formula | Structural formula |
| chloro | chlorine | 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 | tellurocyanogen | tellurocyanic | tellurocyanate | ~ TeCN | ~ Te−C≡N | ||
| azide | hexazine | hydrazoic | azide | azido- | -yl azide | ~ N3 | ~ −≡N ↕ ~ N |
| nitrogen dioxide | dinitrogen tetroxide | nitrous | nitrite | nitro- | -yl nitrite | ~ NO2 | |
| cobalt carbonyl | dicobalt octacarbonyl | cobalt tetracarbonyl hydride | tetracarbonylcobaltate | ? | ? | ~ Co4 | ~ Co4 |
| trinitromethanide | hexanitroethane | trinitromethane | trinitromethanide | trinitromethanido- | -yl trinitromethanide | C3− | C3− |
| tricyanomethanide | hexacyanoethane | tricyanomethane | tricyanomethanide | tricyanomethanido- | -yl tricyanomethanide | C3− | C3− |
| trifluoromethanesulfonate | — | triflic acid | trifluoromethanesulfonate, triflate | triflate trilfate | -yl triflate | ~ O3SCF3− | ~ OTf |
Examples of pseudohalogen molecules
Examples of symmetrical pseudohalogens include cyanogen 2, thiocyanogen 2, selenorhodane 2, azidodithiocarbonate 2. Another complex symmetrical pseudohalogen is dicobalt octacarbonyl, Co28. This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co4.Examples of non-symmetrical pseudohalogens, analogous to the binary interhalogen compounds, are cyanogen halides, 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 which form hydracids with hydrogen and form insoluble salts with Ag such as cyanides, cyanates, isocyanates, rhodanides, selenocyanogens, tellurorhodanides and azides.A common complex pseudohalide is a tetracarbonylcobaltate. The acid HCo4 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, and they can react with metals to form compounds like MX.
Nanoclusters of aluminium are sometimes considered to be pseudohalides since they, too, behave chemically as halide ions, forming Al13I2− and similar compounds. This is due to the effects of metallic bonding on small scales.