Basic Description of the Electronic Structure

The effeet of the ligand field on a five-eoordinate metal has been deseribed by Solomon et al. [44]. The results presented in [44] were obtained for an Fe with four equatorial oxygen ligands and a unique axial ligand. The symmetiy of this erystal field also applies to a five-eoordinate niekel. A sehematie overview of a 

In this ligand field, the g-tensor is characterized by one small principal g-value (gz ge) and two larger g-values g (i = x, y). The g tensor components of the d state can be expressed in terms of the spin-orbit-coupling parameter X at Ni (note that 0 [45]), and the energies of the d orbitals  

The gx and gy values are degenerate (axial symmetry) in a ligand field with C4v symmetry but are expected to become unequal in the [NiFe] hydrogenase, due to the asymmetric protein environment. A rhombic g tensor is observed for all hydrogenases. 

The above-described ligand field also holds for the Ni (d ) and Ni (d ) valence states. Ni is paramagnetic 1/2), and has a d, ground state, for which the g tensor components are 

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