In all known examples of metal–ligand (M–L) δ and φ bonds, the metal orbitals are aligned to the ligand orbitals in a “head-to-head” or “side-to-head” fashion. Here, we report two fundamentally new types of M–L δ and φ interactions; “head-to-side” δ and “side-to-side” φ back-bonding, found in complexes of metallacyclopropenes and metallacyclocumulenes of actinides (Pa–Pu) that makes them distinct from their corresponding Group 4 analogues. In addition to the known Th and U complexes, our calculations include complexes of Pa, Np, and Pu. In contrast with conventional An–C bond decreasing, due to the actinide contraction, the An–C distance increases from Pa to Pu. We demonstrate that the direct L–An σ and π donations combined with the An–L δ or φ back-donations are crucial in explaining this non-classical trend of the An–L bond lengths in both series, underscoring the significance of these δ/φ back-donation interactions, and their importance for complexes of Pa and U in particular.

在所有已知的金属-配体（M-L）δ和φ键的例子中，金属轨道以“头对头”或“侧对头”的方式与配体轨道对齐。在这里，我们报告了两种全新类型的 M-L δ和φ相互作用；在金属环丙烯和锕系金属环积烯 (Pa-Pu) 的复合物中发现了“头对侧” δ和“侧对侧” φ背键，这使得它们与相应的第 4 族类似物不同。除了已知的 Th 和 U 配合物之外，我们的计算还包括 Pa、Np 和 Pu 的配合物。与传统的 An-C 键减少相反，由于锕系收缩，An-C 距离从 Pa 到 Pu 增加。我们证明，直接 L-An σ和π捐赠与 An-L δ或φ反向捐赠对于解释两个系列中 An-L 键长的这种非经典趋势至关重要，强调了这些δ的重要性/ φ回馈相互作用，及其对于 Pa 和 U 复合物的重要性。