Pseudohalogen molecules (meaning "fake" halogens) are inorganic molecules of the general forms Ps–Ps or Ps–X, where Ps is a pseudohalogen group such as cyanide, cyanate, thiocyanate and others, and X is a "true" halogen. Not all combinations are known to be stable.
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.
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.