The 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||Molecule||Acid||Pseudohalide||Ligand name||Organic suffix||Formula||Structural formula|
|chlorine||chlorine||hydrochloric||chloride||chlorido- (IUPAC), chloro-||-yl chloride||~ Cl||~ Cl|
|cyano||cyanogen||hydrocyanic, prussic||cyanide||cyanido- (IUPAC), cyano-||-nitrile||~ CN||~ C≡N|
|cyanate||—||isocyanic||cyanate||cyanato-||-yl cyanate, -yl isocyanate||~ OCN||~ O−C≡N|
|thiocyanate, rhodanide||thiocyanogen||thiocyanic||thiocyanate||thiocyanato-||-yl thiocyanate, -yl isothiocyanate||~ SCN||~ S−C≡N|
|azide||—||hydrazoic||azide||azido-||-yl azide||~ N3||~ N−
|cobalt carbonyl||dicobalt octacarbonyl||cobalt tetracarbonyl hydride||tetracarbonylcobaltate(1−)||?||?||~ Co(CO)4||~ Co(−C≡O)4|
|trinitromethanide||trinitromethane||trintriomethanic||trinitromethanide||trinitromethanido- (IUPAC), trinitromethanido-||-yl trinitromethanide||C(NO2)3-||C(NO2)3-|
|tricyanomethanide||tricyanomethane||tricyanomethanic||tricyanomethanide||tricyanomethanido- (IUPAC), tricyanomethanido-||-yl tricyanomethanide||C(CN)3-||C(CN)3-||
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.
Not all combinations are known to be stable.
Pseudohalides are the anions (or functional groups) of corresponding pseudohalogen groups such as cyanides, cyanates, isocyanates, rhodanides (i.e. thiocyanates and isothiocyanates), selenocyanogens, tellurorhodanides and azides.
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.