Intensity stealing

A mistake often made by those new to the subject is to say that The Laporte rule is irrelevant for tetrahedral complexes (say) because they lack a centre of symmetry and so the concept of parity is without meaning . This is incorrect because the light operates not upon the nuclear coordninates but upon the electron coordinates which, for pure d ox p wavefunctions, for example, have well-defined parity. The lack of a molecular inversion centre allows the mixing together of pure d and p ox f) orbitals the result is the mixed parity of the orbitals and consequent non-zero transition moments. Furthermore, had the original statement been correct, we would have expected intensities of tetrahedral d-d transitions to be fully allowed, which they are not. 

Occasionally, some bands which might otherwise be expected to be weak are observed to be quite strong. Two examples are shown in Fig. 4-4. The first shows the electronic spectrum of a solution containing [CoC ] ions in nitromethane. For this cT system, we expect three spin-allowed transitions and these are observed at roughly 3500, 7000 and 14,000 cm h They correspond (see Chapter 3) to the excitations M2 — Ti F) and T P) respectively. Note, however, that the 

The second example in Fig. 4-4 shows how a (spin-allowed or spin-forbidden) band lying close to a charge transfer band may acquire unusually high intensity. We shall discuss charge-transfer bands more in Chapter 6. For the moment, we note that they involve transitions between metal d orbitals and ligands, are often fully allowed and hence intense. On occasion, the symmetry of a charge transfer state 

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Figure 4-4. a) Spectrum of [CoC J and b) Spectrum showing intensity stealing from a charge-transfer band at higher energy. 

Spin-allowed, Laporte forbidden, with d-p mixing (tetrahedral symmetry) 100 Spin-allowed, Laporte forbidden, but with intensity stealing 1000... 

Abnormal Infrared Effects, (AIRE), Surface-enhanced Infrared Spectroscopy, (SEIRAS), and Intensity Stealing Effects... 

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