Cobalt unfettered by its ligand field Applied magnetic fields induce a field in any compound with unpaired electrons. However, for the induced field to persist once the applied field is gone, the electrons must be configured to manifest orbital angular momentum. Generally, the influence of ligands severely restricts that property in transition metal complexes. Bunting et al. now show that a cobalt ion is just barely affected by two linearly coordinated carbon ligands and, as such, exhibits maximal orbital angular momentum. Although its magnetic properties mainly pertain at very low temperature, its structure offers a more general design principle. Science, this issue p. eaat7319 Magnetic properties arise in a cobalt ion because the influence of two linearly coordinated carbon ligands is unusually weak. INTRODUCTION The magnetic properties of a single metal center are determined by a combination of its total spin S and orbital angular momentum L. Orbital angular momentum gives rise to magnetic anisotropy, an essential property for applications such as information storage and high-coercivity magnets. Unquenched L arises from an odd number of electrons in degenerate orbitals and is typically observed only for free ions, as well as for complexes of the f elements. For the majority of transition metal ions, however, orbital angular momentum is quenched by the ligand field, which removes the requisite orbital degeneracies. Maximal L for a transition metal (L = 3) would require an odd number of electrons in two sets of degenerate orbitals. Such a species would entail a non-Aufbau configuration, wherein the electrons do not fill the d orbitals in the usual order of lowest to highest in energy, and likely exhibit a large magnetic anisotropy. RATIONALE Previous efforts have identified the utility of linear coordination environments for isolating iron complexes with unquenched orbital angular momentum and large magnetic anisotropies. Crucially, transition metals in this environment are unaffected by Jahn-Teller distortions that would otherwise remove orbital degeneracies in the case of partially filled d orbitals. Separately, cobalt atoms deposited on a MgO surface—for which one-coordination of the metal is achieved, provided a vacuum is maintained—were shown to have L = 3, giving rise to near-maximal magnetic anisotropy. Calculations on the hypothetical linear molecule Co(C(SiMe3)3)2 (where Me is methyl) also predicted that this system would possess a ground state with L = 3. Empirically, maximal L in a transition metal complex thus requires both a linear coordination environment and a sufficiently weak ligand field strength to allow for non-Aufbau electron filling. RESULTS The strongly reducing nature of the carbanion ligand hinders isolation of dialkyl cobalt(II) complexes. However, reducing the basicity of the central carbanion through the use of electron-withdrawing aryloxide groups allowed for the synthesis of the dialkyl cobalt(II) complex Co(C(SiMe2ONaph)3)2, where Naph is a naphthyl group. Ab initio calculations on this complex predict a ground state with S = 3/2, L = 3, and J = 9/2 arising from the non-Aufbau electron configuration (dx2–y2, dxy)3(dxz, dyz)3(dz2)1. Much as for lanthanide complexes, the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play the key roles in determining the electronic ground state. dc magnetic susceptibility measurements reveal a well-isolated MJ = ±9/2 ground state, and simulations of the magnetic data from the calculations are in good agreement with the experimental data. Variable-field far-infrared (FIR) spectroscopy shows a magnetically active excited state at 450 cm−1 that, in combination with calculations and variable-temperature ac magnetic susceptibility experiments, is assigned to the MJ = ±7/2 state. Modeling of experimental charge density maps also suggests a d-orbital filling with equally occupied (dx2–y2, dxy), and (dxz, dyz) orbital sets. As a consequence of its large orbital angular momentum, the molecule exhibits slow magnetic relaxation and, in a magnetically dilute sample, a coercive field of 600 Oe at 1.8 K. CONCLUSION Isolation of Co(C(SiMe2ONaph)3)2 illustrates how an extreme coordination environment can confer an f-element–like electronic structure on a transition metal complex. The non-Aufbau ground state enables realization of maximal orbital angular momentum and magnetic anisotropy near the physical limit for a 3d metal. In this respect, the linear L–Co–L motif may prove useful in the design of new materials with high magnetic coercivity. Linear dialkyl cobalt(II). (A) Molecular structure of Co(C(SiMe2ONaph)3)2. Purple, gray, turquoise, and red spheres represent Co, C, Si, and O, respectively. Hydrogen atoms have been omitted for clarity. (B) Energy diagram depicting the energy and electron occupations of the 3d orbitals. (C) The calculated splitting of the ground 4Φ state by spin-orbit coupling. The red line is the experimentally determined energy of the MJ = ±7/2 state. (D) Variable-field FIR spectra of Co(C(SiMe2ONaph)3)2. The top section shows the applied-field spectra (TB) divided by the zero-field spectrum (T0). (E) Variable-field magnetization data for Co(C(SiMe2ONaph)3)2 and Co0.02Zn0.98(C(SiMe2ONaph)3)2 at 1.8 K. μB, bohr magnetons. Orbital angular momentum is a prerequisite for magnetic anisotropy, although in transition metal complexes it is typically quenched by the ligand field. By reducing the basicity of the carbon donor atoms in a pair of alkyl ligands, we synthesized a cobalt(II) dialkyl complex, Co(C(SiMe2ONaph)3)2 (where Me is methyl and Naph is a naphthyl group), wherein the ligand field is sufficiently weak that interelectron repulsion and spin-orbit coupling play a dominant role in determining the electronic ground state. Assignment of a non-Aufbau (dx2–y2, dxy)3(dxz, dyz)3(dz2)1 electron configuration is supported by dc magnetic susceptibility data, experimental charge density maps, and ab initio calculations. Variable-field far-infrared spectroscopy and ac magnetic susceptibility measurements further reveal slow magnetic relaxation via a 450–wave number magnetic excited state.

中文翻译：

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具有来自非 Aufbau 基态的最大轨道角动量的线性钴 (II) 配合物

钴不受其配体场的约束 外加磁场会在任何具有未配对电子的化合物中产生场。然而，一旦施加的场消失，为了使感应场持续存在，电子必须配置为表现出轨道角动量。通常，配体的影响严重限制了过渡金属配合物的性能。邦廷等人。现在表明钴离子几乎不受两个线性配位的碳配体的影响，因此表现出最大的轨道角动量。虽然它的磁性主要适用于极低的温度，但它的结构提供了更通用的设计原理。科学，这个问题 p。eaat7319 磁性在钴离子中出现，因为两个线性配位的碳配体的影响异常微弱。引言单个金属中心的磁性由其总自旋 S 和轨道角动量 L 的组合决定。轨道角动量产生磁各向异性，这是信息存储和高矫顽力磁铁等应用的基本特性。未淬灭的 L 由简并轨道中的奇数个电子产生，通常仅观察到自由离子以及 f 元素的复合物。然而，对于大多数过渡金属离子，轨道角动量被配体场淬灭，这消除了必要的轨道简并。过渡金属的最大 L (L = 3) 将需要两组简并轨道中的奇数电子。这样的物种将需要非 Aufbau 配置，其中电子不会以能量从低到高的通常顺序填充 d 轨道，并且可能表现出大的磁各向异性。基本原理先前的努力已经确定了线性配位环境用于分离具有未淬灭轨道角动量和大磁各向异性的铁配合物的效用。至关重要的是，这种环境中的过渡金属不受 Jahn-Teller 畸变的影响，否则在部分填充的 d 轨道的情况下，这些畸变会消除轨道简并。另外，沉积在 MgO 表面上的钴原子——在保持真空的情况下实现金属的单配位——被证明具有 L = 3，产生接近最大的磁各向异性。对假设的线性分子 Co(C(SiMe3)3)2（其中 Me 是甲基）的计算也预测该系统将具有 L = 3 的基态。根据经验，过渡金属络合物中的最大 L 因此需要线性配位环境和足够弱的配体场强以允许非 Aufbau 电子填充。结果 碳负离子配体的强还原性阻碍了二烷基钴 (II) 配合物的分离。然而，通过使用吸电子芳氧基降低中心碳负离子的碱度，可以合成二烷基钴 (II) 配合物 Co(C(SiMe2ONaph)3)2，其中 Naph 是萘基。对该复合体的从头算计算预测了 S = 3/2、L = 3 和 J = 9/2 的基态，该基态源自非 Aufbau 电子配置 (dx2–y2, dxy)3(dxz, dyz)3(dz2)1。与镧系元素配合物一样，配体场足够弱，以至于电子间排斥和自旋轨道耦合在确定电子基态方面起着关键作用。直流磁化率测量显示了一个良好隔离的 MJ = ±9/2 基态，计算得出的磁数据模拟与实验数据非常吻合。可变场远红外 (FIR) 光谱显示 450 cm-1 处的磁活性激发态，结合计算和可变温度交流磁化率实验，被指定为 MJ = ±7/2 状态。实验电荷密度图的建模还表明 d 轨道填充具有相同占据的 (dx2–y2, dxy) 和 (dxz, dyz) 轨道集。由于其大的轨道角动量，该分子表现出缓慢的磁弛豫，并且在磁性稀释的样品中，在 1.8 K 下具有 600 Oe 的矫顽场。过渡金属络合物上的电子结构。非 Aufbau 基态能够实现接近 3d 金属物理极限的最大轨道角动量和磁各向异性。在这方面，线性 L-Co-L 基序可能在设计具有高矫顽力的新材料中有用。线性二烷基钴(II)。(A) Co(C(SiMe2ONaph)3)2 的分子结构。紫色、灰色、绿松石色和红色球体分别代表 Co、C、Si 和 O。为清楚起见，已省略氢原子。(B) 描述 3d 轨道的能量和电子占据的能量图。(C) 通过自旋轨道耦合计算的 4Φ 基态分裂。红线是实验确定的 MJ = ±7/2 状态的能量。(D) Co(C(SiMe2ONaph)3)2 的可变场 FIR 光谱。上半部分显示了应用场谱 (TB) 除以零场谱 (T0)。(E) Co(C(SiMe2ONaph)3)2 和 Co0.02Zn0.98(C(SiMe2ONaph)3)2 在 1.8 K. μB，玻尔磁子的变场磁化数据。轨道角动量是磁各向异性的先决条件，尽管在过渡金属配合物中，它通常被配体场淬灭。通过降低一对烷基配体中碳供体原子的碱性，我们合成了钴 (II) 二烷基配合物 Co(C(SiMe2ONaph)3)2（其中 Me 是甲基，Naph 是萘基），其中配体场足够弱，以至于电子间排斥和自旋轨道耦合在确定电子基态中起主导作用。直流磁化率数据、实验电荷密度图和 ab initio 计算支持非 Aufbau (dx2–y2, dxy)3(dxz, dyz)3(dz2)1 电子配置的分配。变场远红外光谱和交流磁化率测量进一步揭示了通过 450 波数磁激发态的缓慢磁弛豫。