Californium (Cf) is currently the heaviest element accessible above microgram quantities. Cf isotopes impose severe experimental challenges due to their scarcity and radiological hazards. Consequently, chemical secrets ranging from the accessibility of 5f/6d valence orbitals to engage in bonding, the role of spin–orbit coupling in electronic structure, and reactivity patterns compared to other f elements, remain locked. Organometallic molecules were foundational in elucidating periodicity and bonding trends across the periodic table^1,2,3, with a twenty-first-century renaissance of organometallic thorium (Th) through plutonium (Pu) chemistry^{4,5,6,7,8,9,10,11,12}, and to a smaller extent americium (Am)¹³, transforming chemical understanding. Yet, analogous curium (Cm) to Cf chemistry has lain dormant since the 1970s. Here, we revive air-/moisture-sensitive Cf chemistry through the synthesis and characterization of [Cf(C₅Me₄H)₂Cl₂K(OEt₂)]_n from two milligrams of ²⁴⁹Cf. This bent metallocene motif, not previously structurally authenticated beyond uranium (U)^14,15, contains the first crystallographically characterized Cf–C bond. Analysis suggests the Cf–C bond is largely ionic with a small covalent contribution. Lowered Cf 5f orbital energy versus dysprosium (Dy) 4f in the colourless, isoelectronic and isostructural [Dy(C₅Me₄H)₂Cl₂K(OEt₂)]_n results in an orange Cf compound, contrasting with the light-green colour typically associated with Cf compounds^{16,17,18,19,20,21,22}.

锎 (Cf) 是目前在微克以上可获得的最重元素。Cf 同位素由于其稀缺性和放射性危害而带来了严峻的实验挑战。因此，从 5 f /6 d价轨道的可及性到参与键合、自旋轨道耦合在电子结构中的作用以及与其他f元素相比的反应模式等化学秘密仍然被锁定。有机金属分子是阐明元素周期表中的周期性和键合趋势的基础^1,2,3，随着 21 世纪有机金属钍 (Th) 通过钚 (Pu) 化学^{4,5,6,7,8 的复兴， 9,10,11,12}和较小程度的镅 (Am)¹³、转化化学认识。然而，与 Cf 化学类似的锔 (Cm) 自 1970 年代以来一直处于休眠状态。在这里，我们通过从 2 毫克²⁴⁹ Cf合成和表征 [Cf(C ₅ Me ₄ H) ₂ Cl ₂ K(OEt ₂ )] _{n来复兴对空气/水分敏感的 Cf 化学。}这种弯曲的茂金属基序，以前在铀 (U) ^14,15之外没有经过结构验证，它包含第一个晶体学特征的 Cf-C 键。分析表明 Cf-C 键主要是离子键，共价键贡献很小。降低 Cf 5 f轨道能量与镝 (Dy) 4 f在无色、等电子和等结构的 [Dy(C ₅ Me ₄ H) ₂ Cl ₂ K(OEt ₂ )] _n中，产生橙色 Cf 化合物，与通常与 Cf 化合物相关的浅绿色形成对比^{16,17,18 ,19,20,21,22}。