Pseudohalogens (also known as halogenoids) 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.

Definition

The pseudohalogen concept was introduced by and in 1925. They defined the word pseudohalogen in order to describe a pattern of halogen-like behavior while avoiding the disputed term radical; it is unclear whether this term had reached its modern sense at the time. They initially applied it to describe the chalcocyanate (cyanate , thiocyanate , selenocyanate , tellurocyanate ), azide (), and cyanide () groups.

  • Ps<sub>2</sub> is a volatile, covalently bonded molecular substance, which is symmetrical (structure ). The Ps group must contain more than one electronegative atom; interpseudohalogens must contain more than two such atoms.
  • Ps<sub>2</sub> reacts with metals M to yield salts M<sup>n+</sup>(Ps<sup>−</sup>)<sub>n</sub> that contain Ps<sup>−</sup> anions, analogous to the ionic halides M<sup>n+</sup>(X<sup>−</sup>)<sub>n</sub>
  • Ps<sup>−</sup> reacts with oxidants to re-form Ps<sub>2</sub>
  • Ps also forms covalent pseudohalides APs<sub>n</sub> analogous to the covalent halides AX<sub>n</sub>
  • In particular, Ps forms covalent compounds and with other halogens X and pseudohalogens Ps' that are analogous to the interhalogens
  • HPs is an acid
  • The salts M<sup>n+</sup>(Ps<sup>−</sup>)<sub>n</sub> are insoluble for M<sup>n+</sup> =
  • Ps<sup>−</sup> forms similar metal complexes to the halogens, such as , the pseudohalogen analog of tetrachloromercurate(II)

Downs and Adams (1973) provide a similar list of criteria, with additional comments as follows:

  • Ps<sub>2</sub> reversibly undergoes alkali-induced disproportionation:
  • Ps<sub>2</sub> adds across double bonds:
  • Ambident pseudohalogens, such as thiocyanate, may form multiple isomeric compounds with other groups, such as methyl thiocyanate and methyl isothiocyanate
  • Some pseudohalogens form trimeric anions analogous to the polyhalides , such as and
  • Hydrogen pseudohalides () are typically weaker acids than the hydrogen halides
  • pseudohalide salts are often sparingly soluble
  • Pseudohalogen–metal complexes often have different stability constants to the analogous halogen–metal complexes

Not all these criteria need be met for Ps to be considered a pseudohalogen. For example, the parent compound of cyanate, a classical pseudohalogen group, has never been prepared. Golub and (1978) instead list azide, cyanide, fulminate , cyanate, thiocyanate, selenocyanate, dicyanamide , and tricyanomethanide .

Examples of pseudohalogen molecules

Examples of symmetrical pseudohalogen compounds (, where Ps is a pseudohalogen) include cyanogen , thiocyanogen and hydrogen peroxide . Another complex symmetrical pseudohalogen compound is dicobalt octacarbonyl, . This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, .

Examples of non-symmetrical pseudohalogen compounds (pseudohalogen halides , where Ps is a pseudohalogen and X is a halogen, or interpseudohalogens , where are two different pseudohalogens), analogous to the binary interhalogen compounds, are cyanogen halides like cyanogen chloride (), cyanogen bromide (), nitryl fluoride (), nitrosyl chloride () and chlorine azide (), as well as interpseudohalogens like dinitrogen trioxide (), nitric acid () and cyanogen azide ().

Not all combinations of interpseudohalogens and pseudohalogen halides are known to be stable (e.g. sulfanol ).

Pseudohalide ions

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 ().

A common complex pseudohalide is a tetracarbonylcobaltate . The acid cobalt tetracarbonyl hydride 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 ), and they can react with metals M to form compounds like MX (compare sodium chloride NaCl to sodium azide ).

In addition to cyanate and thiocyanate, a range of other chalcogen and pnictogen analogs have been prepared or studied theoretically.

Pseudohalides as covalent ligands

Many non-symmetric pseudohalides can bond covalently to give two isomeric products. For example, the conjugate acids of cyanate are cyanic acid (H–OCN) and isocyanic acid (H–NCO), which are tautomers of each other. The thiocyanate complexes and are linkage isomers of each other that are distinctly differently colored.

Table of pseudohalogen groups

Many pseudohalogens are known by specialized common names according to where they occur in a compound. The true halogen chlorine is listed for comparison.

{| class="wikitable" style="text-align: center; width:90%;"

|-

! style="width:175px;" | Group

! style="width:175px;" | Dimer

! style="width:175px;" | Hydrogen compound

! Pseudohalide anion

! Ligand name

! In organic compounds

! Formula

! Structural formula

!Ref

|-

! colspan="8" | True halogens

!

|-

| chloro || chlorine || hydrogen chloride || chloride || chlorido-<br />chloro- || -yl chloride || ~&nbsp;Cl ||

|

|-

! colspan="8" | Classical pseudohalogen groups

!

|-

| cyano || cyanogen || hydrogen cyanide || rowspan="2" | cyanide || cyanido-<br />cyano- || -nitrile<br />-yl cyanide || ~&nbsp;CN ||

| || hydrogen isocyanide || isocyanido-<br />isocyano- || -yl isocyanide || ~&nbsp;NC ||

|

|-

| cyanate || never isolated || isocyanic acid || isocyanato- || -yl isocyanate|| ~&nbsp;NCO ||

| || isothiocyanic acid|| isothiocyanato- || -yl isothiocyanate|| ~&nbsp;NCS ||

| || selenocyanic acid|| rowspan="2" | selenocyanate|| selenocyanato- || -yl selenocyanate|| ~&nbsp;SeCN ||

|

|-

| tellurocyanate || tellurocyanogen|| tellurocyanic acid|| rowspan="2" | tellurocyanate|| tellurocyanato- || -yl tellurocyanate|| ~&nbsp;TeCN ||

| || fulminic acid || fulminate || fulminato- || -nitrile oxide&NoBreak; || ~&nbsp;CNO ||

| || never isolated&NoBreak; While they have been hypothesized to be formed during the decomposition of dicyanamide-based ionic liquids, this is yet to be proven. || ~&nbsp; ||

|

|-

| trinitromethanide || hexanitroethane|| trinitromethane (nitroform) || trinitromethanide || trinitromethanido- || -yl trinitromethanide || ~&nbsp; ||

| Gold atoms have the highest electron affinity of any metal, comparable to iodine atoms, and allowing gold to form stable salts in compounds like caesium auride .

:, with structure

There are also some differences between the crystal chemical properties of compared to halides. In particular, the first excitation energy of , corresponding to the promotion of an electron from the 6s orbital to the 6p orbital, should be roughly half of the first excitation energy of , which explains why the anionic character of gold in a formal Au(−I) oxidation state is far more dependent on its local chemical environment.