The Electronic Structure of the Isoelectronic, Square-Planar Complexes [Fe^II(L)₂]^2- and [Co^III(L^Bu)₂]^- (L^2- and (L^Bu)^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(LBu)2]1- (2) both possessing a spin triplet ground state (St = 1) have been investigated by various spectroscopic and density functional methods; H2LBu represents the pro-ligand 3,5-di-tert-butylbenzene-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 d6 (CoIII) or d7 (CoII) 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 Dzz through efficient spin−orbit coupling. Differential covalency effects appear to be of limited importance for this property.