Oxohalide

The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.

This is illustrated by the reaction of a mixture of a chromate or dichromate salt and potassium chloride with concentrated sulfuric acid.

The chromyl chloride produced has no electrical charge and is a volatile covalent molecule that can be distilled out of the reaction mixture.

This gives a favourable enthalpy contribution to the Gibbs free energy change for the reaction[3] Many oxohalides can act as Lewis acids.

[5] Oxohalides of elements in high oxidation states are intensely coloured owing to ligand to metal charge transfer (LMCT) transitions.

[11] Selenium and tellurium form similar compounds and also the oxo-bridged species F5AOAF5 (A = S, Se, Te).

Bromine and iodine are relatively weak oxidizing agents, so fewer oxobromides and oxoiodides are known.

The compounds [Ta2OX10]2− and [M2OCl10]4− (M = W, Ru, Os) have two MX5 groups joined by a bridging oxygen atom.

The unusual linear M−O−M structure can be rationalized in terms of molecular orbital theory, indicating the presence of dπ — pπ bonding between the metal and oxygen atoms.

The crystal structure has a tetragonal symmetry and can be thought of as consisting of layers of Cl−, Bi3+ and O2− ions, in the order Cl-Bi-O-Bi-Cl-Cl-Bi-O-Bi-Cl.

This layered, graphite-like structure results in a relatively low hardness of bismoclite (Mohs 2–2.5) and most other oxohalide minerals.

[23] Mendipite, Pb3O2Cl2, formed from an original deposit of lead sulfide in a number of stages is another example of a secondary oxohalide mineral.

The elements iron, antimony, bismuth and lanthanum form oxochlorides of general formula MOCl.