Colour

    The colours of d-block metal compounds are a characteristic feature of species with ground state electronic configurations other than d⁰ and d¹⁰. For example, [Cr(H₂O)₆]²⁺‏ is skyblue,  [Mn(H₂O)₆]²⁺‏ very pale pink, [Co)H₂O(₆]²⁺‏ pink, [MnO₄]^- intense purple and [CoCl₄]^2- dark blue.  In contrast, salts of Sc(III) (d⁰) or Zn(II) (d¹⁰) are colourless.

    The fact that many of the observed colours are of low intensity is consistent with the colour originating from electronic ‘d–d’ transitions. If we were dealing with an isolated gas-phase ion, such transitions would be forbidden by the Laporte selection rule (equation 1.1 where l is the orbital quantum number). The pale colours indicate that the probability of a transition occurring is low. Table 1.1shows relationships between the wavelength of light absorbed and observed colours.

                            (1.1)

Table 1.1 The visible part of the electromagnetic spectrum.

   The intense colours of species such as [MnO₄]^- have a different origin, namely charge transfer absorptions or emissions .  The latter are not subject to selection rule 1.1 and are always more intense than electronic transitions between different d orbitals.