Transition element complexes Jahn-Teller effects

All divalent and trivalent ions from the first transition series are hexacoordinated. The octahedral coordination of these ions is in agreement with crystal field theory. The interactions between the water molecules and these cations are clearly the result of the cr-donor ligand role of the water molecules, through the overlap of the 3aI MO with the d orbitals of the cation. Hence, the bond exhibits a strong covalent character and the hydrated cations are really coordination complexes. Some distortions of the hydration sphere are observed with Cr and Cu owing to the Jahn-Teller effect. The labile character of the water molecule, when coordinated to transition elements, is very variable (Table 1.2). Some hexa-aquo complexes, which... 

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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