Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell1–3. This is even more pronounced in gadolinium, curium’s lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals4. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons5. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms5,6. Therefore, the question arises of whether the frontier metal orbitals in curium(iii)–ligand interactions can also be modified by applying pressure, and thus be induced to form metal–ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium–sulfur bonds in [Cm(pydtc)4]− (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc)4]− and of a Cm(iii) mellitate that possesses only curium–oxygen bonds. Compared with the changes observed in the [Cm(pydtc)4]− spectra, we observe smaller changes in the f–f transitions in the [Nd(pydtc)4]− absorption spectrum and in the f–f emission spectrum of the Cm(iii) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium–sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds. Enhanced covalency is achieved for a curium complex with curium–sulfur bonds by subjecting the compound to high pressures, indicating that pressure can be used to tune covalency in actinide compounds.

中文翻译：

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压缩吡咯烷二硫代氨基甲酸锔增强共价键

锔在锕系系列中是独一无二的，因为其半填充的 5f 7 壳比其他 5f n 配置具有更低的能量，使其既不具有氧化还原活性，又能够抵抗形成与 5f 壳1-3 接合的化学键。由于 4f 轨道相对于 5f 轨道的收缩，这在钆（锔的镧系元素类似物）中更为明显。然而，在高压下，金属锔会经历从局部到流动 5f 电子的转变。这种转变伴随着由锔原子之间的磁性相互作用决定的晶体结构5,6。因此，问题是锔(iii)-配体相互作用中的前沿金属轨道是否也可以通过施加压力来修饰，从而诱导形成具有一定程度共价键的金属-配体键。在这里，我们报告了在高压（高达 11 吉帕斯卡）下 [Cm(pydtc)4]-（pydtc，吡咯烷二硫代氨基甲酸酯）中锔-硫键中 5f/6d 轨道相对作用变化的实验和计算证据。我们将这些结果与 [Nd(pydtc)4]- 和仅具有锔-氧键的 Cm(iii) 六苯磺酸盐的光谱进行比较。与在 [Cm(pydtc)4]- 光谱中观察到的变化相比，我们观察到 [Nd(pydtc)4]- 吸收光谱和 Cm 的 f-f 发射光谱中 f-f 跃迁的较小变化(iii) 加压后软化，这与其键性质的较小扰动有关。这些结果表明，金属轨道对锔-硫键的贡献在高压下显着增强，并且 5f 轨道参与在 0 到 11 吉帕斯卡之间加倍。我们的工作意味着即使在处理相同的离子时，锕系元素的共价也是复杂的，但它可以指导配体的选择以研究压力对锕系化合物的影响。通过将化合物置于高压下，具有锔-硫键的锔络合物可增强共价性，这表明压力可用于调节锕系化合物的共价性。