A student adds an excess of concentrated hydrochloric acid to a pink aqueous solution containing [Co(H2O)6]2+[\text{Co}(\text{H}_2\text{O})_6]^{2+}[Co(H2O)6]2+ ions, observing a distinct color change to a deep blue solution.
Which complex ion is responsible for this blue color?
[CoCl6]4−[\text{CoCl}_6]^{4-}[CoCl6]4−
[Co(H2O)2Cl4]2−[\text{Co}(\text{H}_2\text{O})_2\text{Cl}_4]^{2-}[Co(H2O)2Cl4]2−
[CoCl4]2−[\text{CoCl}_4]^{2-}[CoCl4]2−
[CoCl4]2+[\text{CoCl}_4]^{2+}[CoCl4]2+
