# Cadmium chloride

Cadmium chloride is a white crystalline compound of cadmium and chloride, with the formula CdCl$_2$. This salt is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. The crystal structure of cadmium chloride (described below), is a reference for describing other crystal structures. Also known are CdCl$_2$•H$_2$O and the hemipentahydrate CdCl$_2$•2.5H$_2$O.

## Structure
### Anhydrous

Anhydrous cadmium chloride forms a layered structure consisting of octahedral Cd$^{2+}$ centers linked with chloride ligands. Cadmium iodide, CdI$_2$, has a similar structure, but the iodide ions are arranged in a HCP lattice, whereas in CdCl$_2$ the chloride ions are arranged in a CCP lattice.

### Hydrates

The anhydrous form absorbs moisture from the air to form various hydrates. Three of these hydrates have been examined by X-ray crystallography.
Crystallographic data for the 3 hydrates of cadmium chloride :
| Compound | CdCl$_2$·H$_2$O | CdCl$_2$·2.5H$_2$O | CdCl$_2$·4H$_2$O |
| Molar mass (g/mol) | 201.33 | 228.36 | 255.38 |
| Crystal Structure | Orthorhombic | Monoclinic | Orthorhombic |
| Space Group | Pnma | P2$_1$/n | P2$_1$2$_1$2$_1$ |
| Lattice constant a (Å) | 9.25 | 9.21 | 12.89 |
| Lattice constant b (Å) | 3.78 | 11.88 | 7.28 |
| Lattice constant c (Å) | 11.89 | 10.08 | 15.01 |
| β | | 93.5° | |
| Density (g/cm$^3$) | 3.26 | 2.84 | 2.41 |
| Comment | Interconnected CdCl$_3$(H$_2$O) octahederons | | Distorted trans-[CdCl$_2$(H$_2$O)$_4$] octahedrons |

## Chemical properties

Cadmium chloride dissolves well in water and other polar solvents. It is a mild Lewis acid.
    CdCl$_2$ + 2 Cl$^−$ → [CdCl$_4$]$^{2−}$
Solutions of equimolar cadmium chloride and potassium chloride give potassium cadmium trichloride. With large cations, it is possible to isolate the trigonal bipyramidal [CdCl$_5$]$^{3−}$ ion.
Cadmium metal is soluble in molten cadmium chloride, produced by heating cadmium chloride above 568 °C. Upon cooling, the metal precipitates.

## Preparation

Anhydrous cadmium chloride can be prepared by the reaction of hydrochloric acid and cadmium metal or cadmium oxide.
    Cd + 2 HCl → CdCl$_2$ + H$_2$
The anhydrous salt can also be prepared from anhydrous cadmium acetate using hydrogen chloride or acetyl chloride.
Industrially, it is produced by the reaction of molten cadmium and chlorine gas at 600 °C.
The monohydrate, hemipentahydrate, and tetrahydrate can be produced by evaporation of the solution of cadmium chloride at 35, 20, and 0 °C respectively. The hemipentahydrate and tetrahydrate release water in air.

## Uses

Cadmium chloride is used for the preparation of cadmium sulfide, used as "cadmium yellow", a brilliant-yellow stable inorganic pigment.
    CdCl $_2$ + H $_2$S → CdS + 2 HCl
In the laboratory, anhydrous CdCl$_2$ can be used for the preparation of organocadmium compounds of the type R$_2$Cd, where R is an aryl or a primary alkyl. These were once used in the synthesis of ketones from acyl chlorides:
    CdCl $_2$ + 2 RMgX → R $_2$Cd + MgCl $_2$ + MgX $_2$
    R $_2$Cd + 2R'COCl → 2R'COR + CdCl $_2$
Such reagents have largely been supplanted by organocopper compounds, which are much less toxic.
Cadmium chloride is also used for photocopying, dyeing and electroplating.
Like all cadmium compounds, CdCl $_2$ is highly toxic and appropriate safety precautions must be taken when handling it.