Final answer:
The color of complex ions depends on the species surrounding the central ion and the oxidation state of the metal ion. In the case of cobalt(II) ions and chloride ions, the difference in color is due to the replacement of water molecules by chloride ions in the coordination sphere.
Step-by-step explanation:
When cobalt(II) ions form a complex ion with chloride ions in aqueous solution, the color of the complex ion is different from the color of the [Co(H₂O)₆]²⁺ ion. This is because the color of a complex ion depends on the species surrounding the central cobalt ion. In the case of [Co(H₂O)₆]²⁺, the color is determined by the presence of water molecules in the coordination sphere. When chloride ions replace the water molecules in the complex ion, the distances between species change, resulting in a different color.
For example, in the formation of the [Cu(NH₃)₄(H₂O)₂]²⁺ complex ion, ammonia replaces water molecules in the hydrated Cu²+ ion. This substitution causes a dramatic color change from the light blue color of [Cu(H₂O)]²⁺ to the blue-violet color of [Cu(NH₃)₄(H₂O)₂]²⁺.
Overall, the colors of complex ions depend on factors such as the species surrounding the central ion and the oxidation state of the metal ion.