Group Theory for Chemists, 2nd Edition

13

d-d Spectra

Literally the most visible distinction between compounds of the d-block elements and their main group counterparts is the predominance of colour among the former. Colour - arising from electronic transitions which occur in the visible region of the electromagnetic spectrum - is largely associated with d-d transitions, that is between the energy levels (terms) discussed in Chapter 12, though other types of transition e.g. charge transfer are also important. At its simplest, the magenta colour of [Ti(H₂O)₆]^3 + (d¹) arises from the single electron e_g ← t_2g transition at ca. 20,000 cm^− 1 but spectra from other dⁿ configurations are generally less easy to rationalise. For example, we might expect a d² ion such as green [V(H₂O)₆]^{3 + ...}