A few ions are known, consisting of two or more electronegative atoms of which at least one is nitrogen, that have properties similar to those of halide ions. These ions are called pseudohalide ions, Pseudohalide ions are univalent and these form salts resembling halide salts.

The pseudohalide ions are :

Cyanide ions (`CN^(-)`) : Isocyanide ion (`NC^(-)`)

Cyanate ion (`OCN^-`) : Fulminate ion (`ONC^(-)`)

Thiocyanate ions (`SCN^-`) : Isothiocyanate ion (`NCS^(-)`)

Selenocyanate ion (`SeCN^-`) : Tellurocyanate ion (`TeCN^(-)`)

Azide ion (`N_3^(-)`) : Azido carbon disulphide ion (`SCSN_3^(-)`)

As the dimers of halide ions are called halogens, the covalent dimers of the pseudohalide ions are called halogens or halogenoids. The pseudohalogens known are :

Cyanogen `(CN)_2` : Oxycyanogen `(OCN)_2`

Thiocyanogen `(SCN)_2` : Selenocyanogen `(SeCN)_2`

Tellurocyanogen `(TeCN)_2` : Azido carbon disulphide `(SCSN_3)_2`

The best known pseudohalide is `CN^(-)` .This resembles `Cl^(-), Br^(-)` and `I^-` in the following respects :

(i) It forms an acid, `HCN`.

(ii) It can be oxidised to `(CN)_2` molecule.

(iii) It forms insoluble salts with `Ag^(+), Pb^(2+)` and `Hg_2^(2+)`

(iv) It forms large number of complexes similar to halide complexes, e.g. `[Cu (CN)_4]^(2-)` and `[CuCl_4]^(2-)`. `[Co(CN)_6]^(3-)` and `[CoCl_6]^(3-)`

(v) lnter-pseudohalogen compounds `ClCN`, `BrCN` and `ICN` can be formed.

(vi) `AgCN` is insoluble in water but soluble in ammonia like `AgCl`.