Hexachlorometalates

Hexachlorometalates are a class of inorganic anions with the general formula [MX6]2-, in which a central metal M is coordinated by six chloride ligands in an octahedral arrangement. The -2 charge of the anion implies that the metal typically has an oxidation state of +4 in these species. In salts, the hexachlorometalate unit is paired with counterions such as potassium, sodium, cesium, or bulky organic cations, giving rise to a variety of crystalline salts.

Representative members include hexachloroplatinate(IV) [PtCl6]2- and hexachlororuthenate(IV) [RuCl6]2-, with analogous anions known for other metals in the +4 oxidation state, such as iridium and several group 4 and 6 metals. In all of these, the six chloride ligands are terminal, producing a nearly ideal octahedral MCl6 core. Depending on the metal, M–Cl bond lengths and spectroscopic properties vary, but the overall geometry remains octahedral and highly symmetric.

Synthesis of hexachlorometallates typically proceeds by oxidation of a lower-valent metal chloride in a chloride-rich medium or by metathesis involving a preformed [MX6]2- salt, followed by exchange to introduce desired counterions. They are generally stable in concentrated chloride solutions but can be sensitive to reducing conditions or hydrolysis under non-chloride environments.

Hexachlorometallates function as useful precursors in inorganic synthesis, enabling preparation of metal complexes and materials. They appear in crystallographic studies, catalysis research, and analytical contexts, where their well-defined octahedral cores and tunable counterions facilitate systematic investigations of metal–chloride chemistry.