- The tetrachlorocadmate ion is an anionic coordination complex with the formula [CdCl₄]²⁻, consisting of a cadmium(II) center coordinated by four chloride ligands in a tetrahedral geometry.
- Because it carries a –2 charge, it occurs only in salt form, paired with various cations such as alkali metals (e.g., potassium tetrachlorocadmate, K₂[CdCl₄]) or organic ammonium ions. The name “tetrachlorocadmate” generally refers to these salts rather than the free ion, which does not exist in isolation.
- Tetrachlorocadmate salts are generally colorless or pale yellow crystalline solids, though appearance can vary depending on the counterion and hydration state. In the crystalline lattice, the [CdCl₄]²⁻ ion adopts a regular tetrahedral geometry, with Cd–Cl bond lengths typically around 2.45–2.50 Å. Many salts crystallize in the orthorhombic or monoclinic systems, and their density ranges from about 2.5 to over 3.0 g/cm³, depending on composition.
- Tetrachlorocadmate salts can be prepared by reacting cadmium chloride (CdCl₂) with an excess of chloride ions from a suitable source, such as hydrochloric acid or an alkali metal chloride, in aqueous or alcoholic solution. For example:
- CdCl₂ + 2 KCl → K₂[CdCl₄]
- In some cases, organic cations such as tetraethylammonium or imidazolium are used, producing salts that are soluble in organic solvents and useful for specialized applications.
- The tetrachlorocadmate ion is stable in concentrated chloride media, but in dilute aqueous solution it can undergo ligand exchange with water molecules to form aquo complexes such as [CdClₓ(H₂O)₆₋ₓ]^(2−x). In strongly acidic chloride solutions, the tetrahedral coordination remains favored. Many tetrachlorocadmate salts are stable at room temperature but decompose upon strong heating to yield cadmium chloride and chlorine gas.
- Uses and Applications
- Research in coordination chemistry – Tetrachlorocadmate salts are model compounds for studying tetrahedral coordination in d¹⁰ metal centers.
- Precursor to cadmium-containing ionic liquids – Particularly with bulky organic cations.
- Intermediate in cadmium halide chemistry – Can be converted to other cadmium halide complexes or materials.
Because of cadmium’s toxicity, these uses are mostly confined to laboratory research rather than large-scale industry.
- Tetrachlorocadmate salts are highly toxic due to their cadmium content. They can cause acute and chronic poisoning through inhalation, ingestion, or prolonged skin contact. Cadmium exposure is associated with kidney damage, bone demineralization, and carcinogenicity. In addition, the chloride ligands make them corrosive in some conditions, especially in the presence of acids that can liberate hydrogen chloride gas.
- Like all cadmium compounds, tetrachlorocadmates are hazardous to aquatic life and persist in the environment. Disposal is strictly regulated, requiring collection as hazardous waste under international chemical safety standards (e.g., EU REACH, U.S. RCRA). Their transport falls under UN 2570 – Cadmium compounds, n.o.s., hazard class 6.1.
