SPIN-ORBIT MIXING AND NEPHELAUXETIC EFFECTS IN THE ELECTRONIC-SPECTRAOF NICKEL(II)-ENCAPSULATING COMPLEXES INVOLVING NITROGEN AND SULFUR DONORS

A study of spin-orbit mixing and nephelauxetic effects in the electron ic spectra of nickel(II)-encapsulating complexes involving mixed nitro gen and sulfur donors is reported. As the number of sulfur donors is s ystematically varied through the series [Ni(N6-xSx)](2+) (x = 0-6), th e spin-forbidden (3)A(2)g --> E-1(g) and (3)A(2g) --> (1)A(1g) transit ions undergo a considerable reduction in energy whereas the spin-allow ed transitions are relatively unchanged. The [Ni(diAMN(6)sar)](2+) and [Ni(AMN(5)Ssar)](2+) complexes exhibit an unusual band shape for the (3)A(2g) --> T-3(2g) transition which is shown to arise from spin-orbi t mixing of the E spin-orbit levels associated with the E-1(g) and T-3 (2g) states. A significant differential nephelauxetic effect also aris es from the covalency differences between the t(2g) and e(g) orbitals with the result that no single set of Racah B and C interelectron repu lsion parameters adequately fit the observed spectra. Using a differen tial covalency ligand-field model, the spectral transitions are succes sfully reproduced with three independent variables corresponding to 10 Dq and the covalency parameters f(t) and f(e), associated with the t(2 g) and e(g) orbitals, respectively. The small decrease in f(t) from un ity is largely attributed to central-field covalency effects whereas t he dramatic reduction in f(e) with increasing number of sulfur donors is a direct consequence of the increased metal-ligand covalency associ ated with the sulfur donors. Covalency differences between the t(2g) a nd e(g) orbitals also result in larger 10Dq values than those obtained simply from the energy of the (3)A(2g) --> T-3(2g) spin-allowed trans ition.