Conradie Jeanet, Patra Ashis K, Harrop Todd C, Ghosh Abhik
Department of Chemistry and Center for Theoretical and Computational Chemistry, University of Tromsø , 9037 Tromsø, Norway.
Inorg Chem. 2015 Feb 16;54(4):1375-83. doi: 10.1021/ic502287m. Epub 2015 Jan 9.
Density functional theory (in the form of the PW91, BP86, OLYP, and B3LYP exchange-correlation functionals) has been used to map out the low-energy states of a series of eight-coordinate square-antiprismatic (D2d) first-row transition metal complexes, involving Mn(II), Fe(II), Co(II), Ni(II), and Cu(II), along with a pair of tetradentate N4 ligands. Of the five complexes, the Mn(II) and Fe(II) complexes have been synthesized and characterized structurally and spectroscopically, whereas the other three are as yet unknown. Each N4 ligand consists of a pair of terminal imidazole units linked by an o-phenylenediimine unit. The imidazole units are the strongest ligands in these complexes and dictate the spatial disposition of the metal three-dimensional orbitals. Thus, the dx(2)-y(2) orbital, whose lobes point directly at the coordinating imidazole nitrogens, has the highest orbital energy among the five d orbitals, whereas the dxy orbital has the lowest orbital energy. In general, the following orbital ordering (in order of increasing orbital energy) was found to be operative: dxy < dxz = dyz ≤ dz(2) < dx(2)-y(2). The square-antiprism geometry does not lead to large energy gaps between the d orbitals, which leads to an S = 2 ground state for the Fe(II) complex. Nevertheless, the dxy orbital has significantly lower energy relative to that of the dxz and dyz orbitals. Accordingly, the ground state of the Fe(II) complex corresponds unambiguously to a dxy(2)dxz(1)dyz(1)dz(2)(1)dx(2)-y(2)(1) electronic configuration. Unsurprisingly, the Mn(II) complex has an S = 5/2 ground state and no low-energy d-d excited states within 1.0 eV of the ground state. The Co(II) complex, on the other hand, has both a low-lying S = 1/2 state and multiple low-energy S = 3/2 states. Very long metal-nitrogen bonds are predicted for the Ni(II) and Cu(II) complexes; these bonds may be too fragile to survive in solution or in the solid state, and the complexes may therefore not be isolable. Overall, the different exchange-correlation functionals provided a qualitatively consistent and plausible picture of the low-energy d-d excited states of the complexes.
密度泛函理论（采用PW91、BP86、OLYP和B3LYP交换关联泛函形式）已被用于描绘一系列八配位四方反棱柱体（D2d）第一排过渡金属配合物的低能态，这些配合物涉及Mn(II)、Fe(II)、Co(II)、Ni(II)和Cu(II)，并带有一对四齿N4配体。在这五种配合物中，Mn(II)和Fe(II)配合物已被合成，并通过结构和光谱进行了表征，而另外三种尚未知晓。每个N4配体由一对通过邻苯二亚胺单元连接的末端咪唑单元组成。咪唑单元是这些配合物中最强的配体，并决定了金属三维轨道的空间排布。因此，其瓣直接指向配位咪唑氮原子的dx(2)-y(2)轨道在五个d轨道中具有最高的轨道能量，而dxy轨道具有最低的轨道能量。一般来说，发现以下轨道排序（按轨道能量增加顺序）起作用：dxy < dxz = dyz ≤ dz(2) < dx(2)-y(2)。四方反棱柱几何结构不会导致d轨道之间出现大的能量间隙，这导致Fe(II)配合物的基态为S = 2。然而，dxy轨道相对于dxz和dyz轨道具有显著更低的能量。因此，Fe(II)配合物的基态明确对应于dxy(2)dxz(1)dyz(1)dz(2)(1)dx(2)-y(2)(1)电子构型。不出所料，Mn(II)配合物具有S = 5/2基态，且在基态1.0 eV范围内没有低能d-d激发态。另一方面，Co(II)配合物既有一个低能的S = 1/2态，也有多个低能的S = 3/2态。预测Ni(II)和Cu(II)配合物具有非常长的金属-氮键；这些键可能过于脆弱，无法在溶液或固态中存在，因此这些配合物可能无法分离得到。总体而言，不同的交换关联泛函为配合物的低能d-d激发态提供了定性一致且合理的图景。
