Spin-orbit mixing and nephelauxetic effects in the electronic spectra of nickel(II)-encapsulating complexes involving nitrogen and sulfur donors

A study of spin−orbit mixing and nephelauxetic effects in the electronic spectra of nickel(II)-encapsulating complexes involving mixed nitrogen and sulfur donors is reported. As the number of sulfur donors is systematically varied through the series [Ni(N6-xSx)]2+ (x = 0−6), the spin-forbidden 3A2g → 1Eg and 3A2g → 1A1g transitions undergo a considerable reduction in energy whereas the spin-allowed transitions are relatively unchanged. The [Ni(diAMN6sar)]2+ and [Ni(AMN5Ssar)]2+ complexes exhibit an unusual band shape for the 3A2g → 3T2g transition which is shown to arise from spin−orbit mixing of the E spin−orbit levels associated with the 1Eg and 3T2g states. A significant differential nephelauxetic effect also arises from the covalency differences between the t2g and eg orbitals with the result that no single set of Racah B and C interelectron repulsion parameters adequately fit the observed spectra. Using a differential covalency ligand-field model, the spectral transitions are successfully reproduced with three independent variables corresponding to 10Dq and the covalency parameters ft and fe, associated with the t2g and eg orbitals, respectively. The small decrease in ft from unity is largely attributed to central-field covalency effects whereas the dramatic reduction in fe with increasing number of sulfur donors is a direct consequence of the increased metal−ligand covalency associated with the sulfur donors. Covalency differences between the t2g and eg orbitals also result in larger 10Dq values than those obtained simply from the energy of the 3A2g → 3T2g spin-allowed transition