Isotropic NMR shifts in transition metal complexes: The calculation of the fermi contact and pseudocontact terms

The Fermi contact and pseudocontact contribution to isotropic NMR shifts in paramagnetic complexes are considered for the following cases, for which formulae customarily used are not strictly applicable: (1) there is an appreciable contribution to the magnetic moment of the complex from unquenched orbital angular momentum; (2) there is appreciable orbital contribution (induced by spin-orbit effects) from spin density at the ligand nucleus; (3) there is appreciable mixing of the ground electronic state and thermally populated excited states by the applied magnetic field. An approximate density matrix treatment is developed to handle these situations. The following examples are discussed as illustrations: octahedral ( t 2 g ) 1 and ( t 2 g ) 5 complexes and complexes in which zero-field splitting occurs.