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 Lothar Birckenbach and Karl Kellermann 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 OCN, thiocyanate SCN, selenocyanate SeCN, tellurocyanate TeCN), azide (N3), and cyanide (CN) groups. Cotton and Wilkinson (1968) give the following criteria for Ps to be an ideal pseudohalogen group:
Ps2 is a volatile, covalently bonded molecular substance, which is symmetrical (structure Ps−Ps). The Ps group must contain more than one electronegative atom; interpseudohalogens Ps−Ps' must contain more than two such atoms. Ps2 reacts with metals M to yield salts Mn+(Ps−)n that contain Ps− anions, analogous to the ionic halides Mn+(X−)n Ps− reacts with oxidants to re-form Ps2 Ps also forms covalent pseudohalides APsn analogous to the covalent halides AXn In particular, Ps forms covalent compounds Ps−X and Ps−Ps' with other halogens X and pseudohalogens Ps' that are analogous to the interhalogens X−X' HPs is an acid The salts Mn+(Ps−)n are insoluble for Mn+ = Ag+, Hg2+2, Pb2+ Ps− forms similar metal complexes to the halogens, such as HgPs2−4, the pseudohalogen analog of tetrachloromercurate(II) Downs and Adams (1973) provide a similar list of criteria, with additional comments as follows:
Ps2 reversibly undergoes alkali-induced disproportionation: Ps2 + 2OH− ⇌ Ps− + OPs− + H2O Ps2 adds across double bonds: Ps2 + CH2=CH2 → PsCH2CH2Ps Ambident pseudohalogens, such as thiocyanate, may form multiple isomeric compounds with other groups, such as methyl thiocyanate CH3−SCN and methyl isothiocyanate CH3−NCS Some pseudohalogens form trimeric anions analogous to the polyhalides X−3, such as (SeCN)−3 and [I(SCN)2]− Hydrogen pseudohalides (HPs) are typically weaker acids than the hydrogen halides Ag+, Hg2+2, Pb2+ 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 (−OCN)2 of cyanate, a classical pseudohalogen group, has never been prepared. Conversely, if the class of pseudohalogen groups is widened to include any univalent group possessing several of the properties above, it becomes extremely broad, encompassing groups such as nitryl (NO2), thiyl (RS), hydroxy (HO), fluorosulfate (FSO3), and perchloryl (ClO3). Cotton and Wilkinson consider the most important pseudohalides to be cyanide, thiocyanate, selenocyanate, azide, cyanate, and CS2N−3 (whose structure was later determined to be that of the substituted heterocycle 1,2,3,4-thiatriazole-5-thiolate, (−S−C(−S−)=N−N=N−)). Golub and Köhler (1978) instead list azide, cyanide, fulminate CNO−, cyanate, thiocyanate, selenocyanate, dicyanamide N(CN)−2, and tricyanomethanide C(CN)−3.
Examples of pseudohalogen molecules 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).
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 (AgN3). A common complex pseudohalide is a tetracarbonylcobaltate [Co(CO)4]−. The acid cobalt tetracarbonyl hydride HCo(CO)4 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 HCo(CO)4), and they can react with metals M to form compounds like MX (compare sodium chloride NaCl to sodium azide NaN3). 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 [(NH3)5Co-SCN]2+ and [(NH3)5Co-NCS]2+ 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.
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