Spin-allowed ligand-field transition

Geometric Dependence of Spin-Allowed Ligand Field Transitions Ligand field theory quantitates the splittings of the one-electron d-orbitals due to their repulsion/antibonding interactions with the ligands. 

We now consider the spin-allowed ligand field transitions of optically active Cu(II) complexes. The table below gives the effect of the L, operator on electrons in d-orbitals.3... 

Luminescence from Cr + complexes, both in the solid state and in solution, is a widespread phenomenon. The great majority belong to type a) in Figure 1, where the luminescent state is 2E and the optical transitions are sharp. The well-known ruby emission is a prototype for this situation. In a weaker ligand field the situation b) in Figure 1 is approached, the T2 state becomes competitive with E as the luminescent state. The T emission, corresponding to a spin-allowed d-d transition, is vibronically broadened. Pure 2 luminescence from Cr + has been observed in halide and oxide coordinations (J ). Intermediate situations with both 2E and 2 emissions ar also known. 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

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. 

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