The electronic structure of the isoelectronic, square-planar complexes [FeII(L)2]2- and [CoIII(LBu)2]- (L2- and (LBu)2- = benzene-1,2-dithiolates):: An experimental and density functional theoretical study

The electronic structures of two formally isoelectronic transition-metal dithiolato complexes [Fe(L)(2)](2-) (1) and [Co(L-Bu)(2)](1-) (2) both possessing a spin triplet ground state (S-t = 1) have been investigated by various spectroscopic and density functional methods; H2LBu represents the pro-ligand 3,5-di-tertbutylbenzene-1,2-dithiol and H2L is the corresponding unsubstituted benzene-1,2-dithiol. An axial zero-field splitting (D) of +32 cm(-1) for 2 has been measured independently by SQUID magnetometry, far-infrared absorption, and variable-temperature and variable-field (VTVH) magnetic circular dichroism spectroscopies. A similar D value of +28 cm(-1) is obtained for 1 on the basis of VTVH SQUID measurements. The absorption spectra of 1 and 2 are found, however, to be very different. Complex 1 is light yellow in color with no intense transition in the visible region, whereas 2 is deep blue. DFT calculations establish that the electronic structures of the [Fe(L)(2)](2-) and [Co(L)(2)](1-) anions are very different and explain the observed differences in their absorption spectra. On the basis of these spectroscopic and theoretical analyses, 1 is best described as containing an intermediate spin Fe(II) ion, whereas for the corresponding cobalt complex, oxidation states describing a d(6)(Co-III) or d(7) (Co-II) electron configuration cannot be unambiguously assigned. The physical origin of the large zero-field splitting in both 1 and 2 is found to be due to the presence of low-energy spin-conserved d-d excitations which lead to a large D-zz through efficient spin-orbit coupling. Differential covalency effects appear to be of limited importance for this property.