Transition metal complexes spin-orbit coupling

The g-value for the free electron, ge, is 2.0023. The approximation made in Equation 13.2 is valid for most discussions of the ESR spectra of the organic free radicals and transition-metal complexes whose orbital angular momentum can be considered to be quenched. Treating the g-value as an experimental quantity does not harm the present discussion, since deviations of g-values from ge can be accounted for by introduction of spin-orbit coupling. 

An increase in the ion annihilation exergonicity AG to values comparable to the excited triplet-state energies (AG I LT < 0) opens an additional electron transfer channel (T-route). In the simplest case, only one excited triplet 3 A or 3 D becomes accessible. Triplet emission can be directly observed from the ECL systems involving rare earth and transition metal complexes with allowed (due to extensive spin-orbit coupling) triplet-singlet electronic transition. 

For the transition metal complexes the position is much different. Each cubic-field term, say, may be split into components by spin-orbit coupling and departure from cubic symmetry and in addition is overlaid by closely coupled vibrations. The result is that for only the simplest ions and... 

There remains the possibility of g-values which depart substantially from 2.00 but are isotropic because of cubic symmetry. In practice such conditions are rare for transition metal complexes, as the Jahn-Teller theorem ensures departure from cubic symmetry in the electronic structure. However, for the lanthanoid and actinoid elements, where the spin—orbit coupling constant is very much larger than kT, the Jahn—Teller theorem may not be relevant and effective cubic symmetry certain. For the lanthanoids, g-values often depart considerably from 2.00, although some anisotropy arising from ligand field splittings is common. For the actinoids, direct observation of ESR is less common but there is evidence of a similar situation. 

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