Pseudohalogen

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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 PsPs 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[edit]

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–N+≡N ↔ –N=N+=N
cobalt carbonyl dicobalt octacarbonyl cobalt tetracarbonyl hydride  ?  ?  ? –Co(CO)4 –Co(–C≡O)4

Examples of pseudohalogen molecules[edit]

Examples of symmetrical pseudohalogens (PsPs) 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.

Examples of non-symmetrical pseudohalogens (Ps–X) are cyanogen halides (ICN, ClCN, BrCN), and other compounds. Sometimes nitrosyl chloride NOCl also is considered as pseudohalogen.

Not all combinations are known to be stable.

Pseudohalides[edit]

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.

A common complex pseudohalide is tetracarbonylcobaltate (Co(CO)4). The acid HCo(CO)4 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 (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.

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