Spin-orbit coupling, 16 lanthanoids

Values for the spin-orbit coupling constants for ions of the transition, lanthanoid and actinoid series are listed in Appendix 3. 

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. 

The spectroscopic and magnetic properties of actinoids are complicated and we mention them only briefly. Absorptions due to 5/-5/ transitions are weak, but they are somewhat broader and more intense (and considerably more dependent on the ligands present) than those due to 4f-4f transitions. The interpretation of electronic spectra is made difficult by the large spin-orbit coupling constants (about twice those of the lanthanoids) as a result of which the RusseU-Saunders coupling scheme partially breaks down. 

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