Cobalt CoCl4 2‘ blue

Transition metals have a particular tendency to form complex ions because they have more than one oxidation state. This property allows them to act effectively as Lewis acids in reactions with many molecules or ions that serve as electron donors, or as Lewis bases. For example, a solution of cobalt(II) chloride is pink because of the presence of the Co(H20)6 ions (Figure 16.8). When HCl is added, the solution turns blue as a result of the formation of the complex ion CoCl4 ... 

Figure 16.10 (Left) An aqueous cobalt(ll) chloride solution. The pink color Is due to the presence of CofMaQ) ions. (Right) After the addition of HCl solution, the solution turns blue because of the formation of the complex CoCl4 ions.

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