Electronic absorption spectra of octahedral and tetrahedral complexes

Electronic absorption spectra of octahedral and tetrahedral complexes 

Electronic spectroscopy is a complicated topic and we shall restrict our discussion to high-spin complexes. This corresponds to the weak field limit. We begin with the electronic absorption spectrum of an octahedral (fi ion, exemplified by [Ti(OH2)6] . The spectrum of [Ti(OH2)e] + (Fig. 20.4) exhibits one broad band. However, close inspection shows the presence of a shoulder indicating that the absorption is actually two closely spaced bands (see below). The term symbol for the ground state of Ti (d, one electron with L = 2, S = 5 ) is D. In an octahedral field, this is split into Fig. 20.18 Energy level diagram for a t/ ion in an octahedral field. 

For the configuration (e.g. Cu ) in an octahedral field (actually, a rare occurrence because of Jahn-TeUer effects which lower the symmetry), the ground state of the free ion D) is again split into Tig and terms, but, in 

Finally, Fig. 20.19 shows that for each of the octahedral and tetrahedral d, d, d and d ions, only one electronic transition from a ground to excited state is possible  

In order to work out the terms for the cF configuration, a table of microstates (Table 20.7) must be constmcted. However, for interpreting electronic spectra, the table can be simplified because we need concern ourselves only with the terms of maximum spin multiplicity. This corresponds to a weak field limit. For the d ion, we therefore focus on the i and P (triplet) terms. These are summarized in Table 20.9, with the corresponding microstates represented only in terms of electrons with mg = +j. It follows from the mles given in Section 20.6 that the p term is expected to be lower in energy than the P term. In an octahedral field, the P term does not split, and is labelled Pig. 

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