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.本研究针对两种形式上等电子的过渡金属二硫醇配合物——[Fe(L)₂]²⁻（标记为1）与[Co(LBu)₂]¹⁻（标记为2）——开展了电子结构研究，二者均具有自旋三重态基态（St=1）；其中H₂LBu代表配体前体3,5-二叔丁基邻苯二硫酚，H₂L则为对应的未取代邻苯二硫酚。研究采用了多种光谱学方法与密度泛函理论（density functional theory, DFT）手段。针对配合物2，研究人员通过超导量子干涉仪（Superconducting Quantum Interference Device, SQUID）磁测量法、远红外吸收光谱法以及变温变场（variable-temperature and variable-field, VTVH）磁圆二色谱（magnetic circular dichroism, MCD）等多种手段独立测得其轴向零场分裂（axial zero-field splitting, ZFS）参数D为+32 cm⁻¹；配合物1则通过VTVH-SQUID联合测量得到了相近的D值，为+28 cm⁻¹。然而二者的吸收光谱存在显著差异：配合物1呈浅黄色，可见光区域无强吸收跃迁，而配合物2则呈深蓝色。密度泛函理论计算结果表明，[Fe(L)₂]²⁻与[Co(L)₂]¹⁻阴离子的电子结构差异显著，这也合理解释了二者吸收光谱的观测差异。基于上述光谱学与理论分析，配合物1可被最优描述为包含中间自旋Fe(II)离子；而对于对应的钴配合物，则无法明确归属其氧化态为d⁶构型的Co(III)或是d⁷构型的Co(II)。研究发现，配合物1与2的大零场分裂的物理起源均为低能自旋守恒d-d跃迁通过高效的自旋-轨道耦合（spin-orbit coupling）产生了大的Dzz参数；而共价键差异效应对该性质的影响较为有限。