Pseudo Halogens

Pseudo Halogens

Quite a few inorganic radicals are known which in the free state resemble in properties with the elemental halogens and in the form of anions exhibit reactions analogous to those of the halide ions. Examples are: cyanogens, (CN)2, thiocyanogen, (SCN)2, selenocyanogen (SeCN)2, etc. These are called halogenoids or pseudo−halogens. Excepting thiocyanogen, they are volatile in the free state. Like the halogens, they also form hydracids with H2 and combine with metals to form salts;  their Ag+, Hg(I)− and pb²⁺ −salts are insoluble in water.

Pseudo−halogens are prepared like halogens by oxidation of the hydracids or salts or by decomposition of some Pb(IV)−compounds:

4HSCN + MnO2 Mn(SCN)2 + (SCN)2 + 2H2O

[Similar to 4HCl + MnO2 MnCl2 + Cl2 + 2H2O]

Pb(SCN)4 Pb(SCN)2 + (SCN)2

[Similar to PbCl4 PbCl2 + Cl2]

The oxidizing power of the halogenoids decrease in the order:

(CN)2 > (SCN)2 > (SCN3)2 > (SeCN)2

Thus thiocyanogen would oxidize I− liberating I2

(SCN)2 + 2I− 2SCN− + I2

Inter−halogenoids such as CNN3 and CNSCN are also known which bear analogies with ClF, ICl, IBr etc.