Question
When concentrated $$HCl$$ is added to an aqueous solution of $$CoC{l_2},$$ its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction ?
A.
$${\left[ {CoC{l_4}} \right]^{2 - }}$$
B.
$${\left[ {CoC{l_6}} \right]^{3 - }}$$
C.
$${\left[ {CoC{l_6}} \right]^{4 - }}$$
D.
$${\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$$
Answer :
$${\left[ {CoC{l_4}} \right]^{2 - }}$$
Solution :
Aqueous of solution of $$CoC{l_2}$$ contains $${\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }}$$ which is pinkish in colour so option (D) is incorrect.
Reduction potential of $$C{o^{3 + }} \to C{o^{2 + }}$$ is high so option (B) is incorrect. $$C{o^{2 + }}$$ does not oxidises easily to $$C{o^{3 + }}.$$
It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes. So $${\left[ {CoC{l_4}} \right]^{2 - }}$$ is deep blue in colour.