Complex formation

   d-Block metal ions readily form complexes, with complex formation often being accompanied by a change in colour and sometimes a change in the intensity of colour. Equation 1.1 shows the effect of adding concentrated HCl to aqueous cobalt(II) ions.

                 (1.1)

    The formation of such complexes is analogous to the formation of those of s- and p-block metals and discussed in previous chapters, e.g. [K(18-crown-6)]‏, [Be(H₂O)₄]²⁺‏, trans-[SrBr₂(py)₅], [AlF₆]^3-, [SnCl₆]^2- and [Bi₂(C₆H₄O₂)₄]^2-.