The effects induced by spin-orbit coupling on the electronic states of Ni(II) complexes are studied with ab-initio and crystal field calculations.

The ab-initio calculations are performed in two steps, to separate the coulomb and the spin-orbit effects. The first step is an ordinary Cl calculation which provides excited states that are spin eigenfunctions. The second step is a spin-orbit coupled Cl of a selection of the previously created states. The spin-orbit coupling is described by an effective one-electron operator. Double group symmetry is used throughout to facilitate the analysis, and the experimental comparisons, of the spin-orbit states and the matrix elements.

The method is applied, and compared with experiments, on cubic Oh and orthorhombic Ö2h NiFg-. For the cubic form the calculated excited states are in excellent agreement. For the orthorhombic form comparison of the spin-orbit splitting, or zero field splitting, of the ground state shows that the calculated states are in correct order but a factor of two larger than the experimental energies.

To investigate the dependence of the zero-field splitting on the degree of distortion from octahedral symmetry and the type of ligand, a series of calculations with different ligands at different distances is performed.

Finally the possibility to parametrize the zero-field splitting as a function of the normal coordinate modes in octahedral symmetry is explored with crystal field and ab-initio calculations. The parameters for a generally distorted NiF4/6 complex are given and it is shown that crystal field calculations can be made to reproduce the ab-initio results at the expense of different scalings for the radial and the curvilinear modes.