Cadmium chloride

Cadmium chloride is a white crystalline compound of cadmium and chlorine, with the formula CdCl₂. It is a hygroscopic solid which is highly soluble in water and slightly soluble in alcohol. Although it is considered to be ionic, it has considerable covalent character to its bonding. The crystal structure of cadmium chloride (described below), composed of two-dimensional layers of ions, is often used as a reference for describing other crystal structures.

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Crystal structure

Cadmium chloride forms crystals with cubic symmetry. The structure is based on the sodium chloride crystal structure, but with half of the metal ions removed (the "missing" ions are shown as hatched light blue balls in the drawing) so that the resultant structure consists of a layered lattice. This same basic structure is found in many other salts and minerals, particularly those where there is some degree of covalent bonding, such as in manganese(II) chloride, cobalt(II) chloride or magnesium chloride.

Cadmium iodide, CdI₂, has a very similar crystal structure to CdCl₂. The individual layers in the two structures are identical, but in CdCl₂ the chloride ions are arranged in a CCP lattice, whereas in CdI₂ the iodide ions are arranged in a HCP lattice.

Chemical properties

Cadmium chloride has a high solubility in water, and it dissociates into ions. A certain amount of hydrolysis to species such as [CdOH(H₂O)_x]⁺ may occur. The high solubility may be due in part to formation of complex ions such as [CdCl₄]²⁻ (i.e CdCl₂ is a Lewis acid). With excess chloride ions in water or acetonitrile it forms mainly [CdCl₃]⁻ and the tetrahedral anion, [CdCl₄]²⁻:

CdCl₂(aq) + 2 Cl⁻(aq) → [CdCl₄]²⁻(aq)

With large cations it is possible to isolate the trigonal bipyramidal [CdCl₅]³⁻ ion.

Preparation

Anhydrous cadmium chloride can be prepared by the action of anhydrous chlorine or hydrogen chloride gas on heated cadmium metal.

Cd(s) + 2 HCl(g) → CdCl₂(s) + H₂(g)

Hydrochloric acid may be used to make hydrated CdCl₂ from the metal, or alternatively from cadmium oxide or cadmium carbonate.

Uses

Cadmium chloride is used for the preparation of cadmium sulfide, used as "Cadmium Yellow", a brilliant yellow pigment which is stable to heat and sulfide fumes.

CdCl₂(aq) + H₂S(g) → CdS(s) + 2 HCl(aq)

In the laboratory, anhydrous CdCl₂ can be used for the preparation of organocadmium compounds of the type R₂Cd where R = aryl or primary alkyl. These were once used in the synthesis of ketones from acyl chlorides (see below), but nowadays they have largely been supplanted by organocopper compounds, which are much less toxic.

CdCl₂ + 2 RMgX → R₂Cd + MgCl₂ + MgX₂

R₂Cd + R'COCl → R'COR + CdCl₂

Cadmium chloride is also used for electroplating.

References

1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
4. D. Nicholls, Complexes and First-Row Transition Elements, Macmillan Press, London, 1973.
5. A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
6. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.