When concentrated HCI is added to an aqueous solution of `CoCl_(2) ` . Its colour changes from reddish pink to deep blue . Which complex ion gives blue colour in this reaction ?

Options

A. `[CoCl_(4)]^(2-)`
B. `[CoCl_(6)]^(3-)`
C. `[CoCl_(6)]^(4-)`
D. `[Co(H_(2)O)_(6)]^(2+)`

Answer

A

Explanation

Aqueous solution of `CoCl_2` contains `[Co(H_2O)_6]^(2+)` which is pinkish in color so option (d) is incorrect. Reduction potential of `Co^(3+) rarr Co^(2+)` is high so option (b) is incorrect. `Co^(2+)` does not oxidises easily to ` Co^(3+)` . It is general case that symmetrical substituted octahedral complexes. So `[CoCl_4]^2` is deep blue in colour.