Pseudohalogen

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.^[1] 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.

The pseudohalogen concept was introduced by Lothar Birckenbach and Karl Kellermann in 1925.^[2][3] 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.^{[4]: 12901 [5]: 786} They initially applied it to describe the chalcocyanate (cyanate OCN, thiocyanate SCN, selenocyanate SeCN, tellurocyanate TeCN), azide (N₃), and cyanide (CN) groups.^[3][5]

Cotton and Wilkinson (1968) give the following criteria for Ps to be an ideal pseudohalogen group:^[6]

• 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 APs_n analogous to the covalent halides AX_n
• 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:^[2]

• 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.^[2][6] 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).^[2]

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−)).^[6][7] Golub and Köhler (1978) instead list azide, cyanide, fulminate CNO⁻, cyanate, thiocyanate, selenocyanate, dicyanamide N(CN)−2, and tricyanomethanide C(CN)−3.^[8][9]: 15

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).

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).

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

Gold displays halogen-like behavior in certain respects, which have sometimes led it to be described as a 'pseudo-halogen', albeit its chemical properties are clearly distinct from the halogens and classical pseudohalogens.^{[21]: 112 [22]} Gold atoms have the highest electron affinity of any metal, comparable to iodine atoms, and allowing gold to form stable Au⁻ salts in compounds like caesium auride CsAu.^[23] Gold also undergoes halogen-like disproportionation to Au(I) and Au(−I) when heated with alkali metals (Cs, Rb, K) together with their oxides, in reactions such as:^[23][24]

3Cs + 5Au + 2Cs₂O → Cs₇Au₅O₂, with structure [Cs⁺Au⁻]₄[(Cs⁺)₃(AuO₂)³⁻]

There are also some differences between the crystal chemical properties of Au⁻ compared to halides. In particular, the first excitation energy of Au⁻, 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 I⁻, which explains why the anionic character of gold in a formal Au(−I) oxidation state is far more dependent on its local chemical environment.^[23]

• Superhalogen