The actinides, from californium to nobelium (Z = 98–102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing Cf^II materials, but investigations are hampered by the fact that they have remained difficult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce Cf^III to Cf°. Here we show that a Cf^II crown–ether complex, Cf(18-crown-6)I₂, can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf^III can be quantitatively reduced to Cf^II, and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf^II and Cf^III complexes without the Al/Hg amalgam. Quantum-chemical calculations show that the Cf‒ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.

锕系元素，从锎到铌 (Z = 98–102)，已知具有可接近的 +2 氧化态。了解这种化学行为的起源需要表征 C​​f ^II材料，但由于它们仍然难以分离这一事实阻碍了研究。这部分是由于操纵这种不稳定元素的内在挑战，以及缺乏不会将 Cf ^III还原为 Cf° 的合适还原剂。在这里，我们展示了 Cf ^II冠醚络合物 Cf(18-crown-6)I ₂可以使用 Al/Hg 汞合金作为还原剂来制备。光谱证据表明 Cf ^III可以定量还原为 Cf ^{II，并且溶液中的快速辐射再氧化产生不含 Al/Hg 汞齐的 Cf II}和 Cf ^III复合物的共结晶混合物。量子化学计算表明，Cf-配体相互作用是高度离子化的，并且不存在 5 f /6 d混合，导致 5 f → 5 f跃迁较弱，吸收光谱由 5 f → 6 d跃迁主导。