Glaser T, Rose K, Shadle S E, Hedman B, Hodgson K O, Solomon E I
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Am Chem Soc. 2001 Jan 24;123(3):442-54. doi: 10.1021/ja002183v.
X-ray absorption spectroscopy (XAS) at the sulfur ( approximately 2470 eV) and chlorine ( approximately 2822 eV) K-edges has been applied to a series of 4Fe-4S model complexes. These are compared to 2Fe-2S model complexes to obtain insight into the localized ground state in the mixed-valence dimer versus the delocalized ground state in the mixed-valence tetramer. The preedges of hypothetical delocalized mixed-valence dimers Fe(2)S(2) are estimated using trends from experimental data and density functional calculations, for comparison to the delocalized mixed-valence tetramer Fe(4)S(4). The differences between these two mixed-valence sites are due to the change of the sulfide-bridging mode from micro(2) to micro(3). The terminal chloride and thiolate ligands are used as spectator ligands for the electron density of the iron center. From the intensity of the preedge, the covalency of the terminal ligands is found to increase in the tetramer as compared to the dimer. This is associated with a higher effective nuclear charge on the iron in the tetramer (derived from the energies of the preedge). The micro(3)-bridging sulfide in the tetramer has a reduced covalency per bond (39%) as compared to the micro(2)-bridging sulfide in the dimer (51%). A simple perturbation model is used to derive a quadratic dependence of the superexchange coupling constant J on the covalency of the metal ions with the bridging ligands. This relationship is used to estimate the superexchange contribution in the tetramer (J = -156 cm(-)(1)) as compared to the mixed-valence dimer (J = -360 cm(-)(1)). These results, combined with estimates for the double exchange and the vibronic coupling contributions of the dimer sub-site of the tetramer, lead to a delocalized S(t) = (9)/(2) spin ground state for the mixed-valence dimer in the tetramer. Thus, the decrease in the covalency, hence the superexchange pathway associated with changing the bridging mode of the sulfides from micro(2) to micro(3) on going from the dimer to the tetramer, significantly contributes to the delocalization of the excess electron over the dimer sub-site in the tetramer.
硫（约2470电子伏特）和氯（约2822电子伏特）K边的X射线吸收光谱（XAS）已应用于一系列4Fe-4S模型配合物。将这些与2Fe-2S模型配合物进行比较，以深入了解混合价二聚体中的局域基态与混合价四聚体中的离域基态。使用实验数据和密度泛函计算的趋势估计假设的离域混合价二聚体[Fe(2)S(2)]⁺的前缘，以便与离域混合价四聚体[Fe(4)S(4)]²⁺进行比较。这两个混合价位点之间的差异是由于硫化物桥连模式从μ₂变为μ₃。末端氯和硫醇盐配体用作铁中心电子密度的旁观配体。从前缘强度发现，与二聚体相比，四聚体中末端配体的共价性增加。这与四聚体中铁上更高的有效核电荷有关（从前缘能量得出）。与二聚体中的μ₂桥连硫化物（51%）相比，四聚体中的μ₃桥连硫化物每个键的共价性降低（39%）。使用一个简单的微扰模型得出超交换耦合常数J对金属离子与桥连配体共价性的二次依赖性。与混合价二聚体（J = -360厘米⁻¹）相比，该关系用于估计四聚体中的超交换贡献（J = -156厘米⁻¹）。这些结果，结合对四聚体中二聚体子位点的双交换和振动耦合贡献的估计，导致四聚体中混合价二聚体的离域S(t) = 9/2自旋基态。因此，共价性的降低，即与从二聚体到四聚体时硫化物桥连模式从μ₂变为μ₃相关的超交换途径，显著促进了四聚体中二聚体子位点上多余电子的离域。
