Laporte-allowed ligand-metal transitions

Charge-transfer excitations from odd ligand levels to the even metal ys and y>3 levels clearly represent formally Laporte-allowed u - -g transitions, and consequently should be intense. Ligand to metal transitions involving even ligand orbitals are of course also possible, but would be parity forbidden and are therefore rather seldom observed. For many of the ions here treated though the data are derived from reflectance measurements and the intensity criterion is of limited value because of the increase in the scattering coefficient which usually occurs above about 25 kK. [c.f. (7)]. 

E20.22 The blue-green colour of the Cr ions in [Cr(H20) ] is caused by spin-allowed but Laporte-forbidden ligand field transitions. The relatively low-molar-absorption coefficient, , which is a manifestation of the Laporte-forbidden nature of the transitions, is the reason that the intensity of the colour is weak. The oxidation state of chromium in tetrahedral chromate dianion is CifVI), which is d . Therefore, no ligand field transitions are possible. Ilte intense yellow colour is due to LMCT transitions (i.e., electron transfer from the oxide ion ligands to the Cr(VI) metal centre). Charge transfer transitions are intense because they are both spin-allowed and Laporte-allowed. 

The first intense (Laporte-allowed) electron transfer absorption band of a complexed ion is generally due to the transition of an electron essentially from a ligand orbital to an orbital of the central ion for an octahedral complex of a transition metal, for example, the transition is from the (tt + a) orbital set to the (d) t2g or (d) e. The energy of this absorption band has been correlated, by Jt rgensen (7), with the electronegativity difference between the surrounding ligands and the central ion such that... 

Experimentally, spin-allowed d-d bands (we use the quotation marks again) are observed with intensities perhaps 100 times larger than spin-forbidden ones but still a few orders of magnitude (say, two) less intense than fully allowed transitions. This weakness of the d-d bands, alluded to in Chapter 2, is a most important pointer to the character of the d orbitals in transition-metal complexes. It directly implies that the admixture between d and p metal functions is small. Now a ligand function can be expressed as a sum of metal-centred orbitals also (see Box 4-1). The weakness of the d-d bands also implies that that portion of any ligand function which looks like a p orbital when expanded onto the metal is small also. Overall, therefore, the great extent to which d-d bands do satisfy Laporte s rule entirely supports our proposition in Chapter 2 that the d orbitals in Werner-type complexes are relatively well isolated (or decoupled or unmixed) from the valence shell of s and/or p functions. 

The absorption spectroscopy in the UV-Vis-NIR is especially rich for the actinides, allowing for fairly simple determinations of the metal oxidation state. The primary absorption bands result from f f transitions, f d and ligand-to-metal charge transfers. The f — f transitions are typically weak since they are forbidden under the LaPorte selection rules. Distortions in symmetry allow for relaxation in these rules and bands in the visible to near-infrared range result. Complexes that contain an inversion syimnetry, for example Pu02CLt, have weaker f- -f transitions (e < 20 cm ). The direct interactions of the 5f orbitals... 

In dealing with the complexes of metals containing partly filled d shells, it has become customary to label absorption bands as d-d transitions, where the initial and final electronic states can be considered as mainly localized on the metal, and charge-transfer transitions, where one state is mainly localized on the metal and the other on the ligand(s). The classification of spin-allowed and spin-forbidden bands is retained, as are parity restrictions, although these are often termed symmetry restrictions. There are, in addition, several confusing references to the Laporte rule. ... 

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