Cobalt(II) chloride

Cobalt(II) chloride (CoCl₂) is a chemical compound composed of cobalt and chlorine. It is blue when anhydrous, and a deep magenta colour when hydrated- for this reason it is widely used as an indicator for water, for example in drying agents such as Silica gel. The magenta hexahydrate is probably the most familiar cobalt compound in the laboratory:

Cobalt(II) chloride hexahydrate

It is ionic and dissolves readily in water and alcohol, and it has the interesting property that a concentrated aqueous solution is red at room temperature, but becomes blue when heated^[4].

Chemical properties

Since cobalt(II) chloride is soluble in water, it can be used to prepare a variety of cobalt salts, e.g.,

CoCl₂(aq) + K₂S(aq) → CoS(s) + 2 KCl(aq)

The most stable oxidation state for cobalt is +2, so CoCl₂ is generally resistant to oxidation. However, in the presence of ammonia or amines, cobalt(II) chloride is easily oxidised (even by oxygen in the air to give a variety of stable cobalt(III) amine complexes, for example:

4 [Co(H₂O)₆]Cl₂ + 4 NH₄Cl + 20 NH₃ + O₂ → 4 [Co(NH₃)₆]Cl₃ + 26 H₂O

The reaction is performed in the presence of charcoal as a catalyst.

Unlike most complexes, these are very slow to exchange ligands, so they are said to be kinetically inert. The German chemist Alfred Werner was awarded the Nobel prize in 1913 for his studies on a series of these Co(III) compounds, work which led to an understanding of the structures of such "coordination compounds".

Regarding the existence of a simple Co(III) chloride, CoCl₃, the literature is contradictory. Ref. 2 describes such a compound as known, but ref. 1 (which is more recent) states, "Apart from ...CoF₃, ....the only known halides of cobalt are the dihalides."

Cobalt(II) chloride is a weak Lewis acid, reacting with chloride ion to produce salts containing the tetrahedral [CoCl₄]^2- ion. It forms complexes with ligands such as pyridine (see "uses").

Preparation

Cobalt(II) chloride may be prepared in its anhydrous form from cobalt metal and chlorine gas:

Co(s) + Cl₂(g) → CoCl₂(s)

The hydrated form can be prepared from cobalt(II) hydroxide or cobalt(II) carbonate with hydrochloric acid.

Uses

A popular use for cobalt(II) chloride is for the detection of moisture. For example, in the US calcium_sulfate is sold as a drying agent under the trade name Drierite. When cobalt(II) chloride is added as an indicator, the drying agent is blue when still active, pink when exhausted, corresponding to the anhydrous and hydrated forms of CoCl₂, respectively.

In the laboratory, cobalt(II) chloride serves as a starting point for the synthesis of a variety of cobalt compounds. For example, the reaction of 1-norbonyllithium with CoCl₂ produces a brown, thermally stable cobalt(IV) tetralkyl^[5]- the only compound of its kind for which the detailed structure is fully known^[1]:

Reaction of anhydrous CoCl₂ with cyclopentadienylsodium in THF to give dark-purple cobaltocene, which can readily be oxidised to a Co(III) compound, the cobaltacenium cation which is isoelectronic with ferrocene. CoCl₂ also reacts with ligands such as pyridine to produce tetrahedral metal complexes:

CoCl₂ + 2 C₅H₁₀N → [CoCl₂(C₅H₁₀N)₂]

Precautions

Toxic, cancer suspect agent.

Suppliers/Manufacturers

• Aldrich
• Strem
• Fisher
• VWR
• Alfa

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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. A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. E. K. Byrne, D. S. Richeson, K. H. Theopold, Journal of the Chemical Society, Chemical Communications, 1491–2 (1986).
6. Hill, Petrucci, McCreary, Perry, "General Chemistry", 4th ed., Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA.