- The tetrabromocadmate ion has the formula [CdBr₄]²⁻, consisting of a cadmium(II) center tetrahedrally coordinated by four bromide ligands. It carries a –2 charge and is only found in salt form, paired with cations such as alkali metals (e.g., potassium tetrabromocadmate, K₂[CdBr₄]) or organic cations like tetraalkylammonium. The free [CdBr₄]²⁻ ion does not exist in isolation outside a stabilizing ionic lattice.
- Tetrabromocadmate salts are usually colorless to pale yellow crystalline solids. The [CdBr₄]²⁻ ion adopts a tetrahedral geometry, with Cd–Br bond lengths generally around 2.65–2.75 Å, longer than Cd–Cl due to the larger radius of bromide. Depending on the counterion and hydration state, these salts crystallize in various systems (often monoclinic or orthorhombic). They tend to be denser than their chloride analogues, with densities ranging from ~3.0 to over 4.0 g/cm³.
- Tetrabromocadmate salts can be synthesized by dissolving cadmium bromide (CdBr₂) in a bromide-rich medium (such as aqueous or alcoholic KBr or NaBr) to favor the formation of the [CdBr₄]²⁻ complex:
- CdBr₂ + 2 KBr → K₂[CdBr₄]
- Organic cation versions are made similarly by mixing CdBr₂ with the corresponding bromide salt of the desired organic ammonium ion.
- The [CdBr₄]²⁻ ion is stable in concentrated bromide solutions but can undergo ligand exchange with water in dilute or mixed media, leading to species like [CdBrₓ(H₂O)₆₋ₓ]^(2−x). Upon strong heating, tetrabromocadmate salts decompose to cadmium bromide, bromine gas, and possibly cadmium oxide in oxidizing conditions.
- Uses and Applications: Due to cadmium’s toxicity, industrial uses are minimal and controlled.
- Coordination chemistry research – Model compound for tetrahedral d¹⁰ metal halide complexes.
- Precursors to cadmium-based materials – Particularly in halide perovskite and related materials research.
- Ionic liquids and molten salts – When paired with bulky organic cations, used in electrochemical studies.
- Like all cadmium compounds, tetrabromocadmate salts are highly toxic and carcinogenic. They can be harmful via inhalation, ingestion, or skin absorption. Bromide ligands add corrosive potential in acidic conditions, where hydrogen bromide (HBr) gas may be released.
- Tetrabromocadmate salts are hazardous to aquatic ecosystems, persistent in the environment, and subject to strict handling and disposal regulations. Transport is generally classified under UN 2570 – Cadmium compounds, n.o.s., hazard class 6.1.
