δ and φ back-donation in AnIV metallacycles

In all known examples of metal-ligand (M-L) delta and 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 delta and phi interactions; head-to-side delta and side-to-side phi 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 sigma and pi donations combined with the An-L delta or phi back-donations are crucial in explaining this non-classical trend of the An-L bond lengths in both series, underscoring the significance of these delta/phi back-donation interactions, and their importance for complexes of Pa and U in particular. p id= Par Metal-ligand delta and phi interactions, though considered weak, may be necessary for fully describing the electronic and geometric structures of certain compounds. Here, in actinide metallacycles, the authors discover two new types of M-L delta and phi back-bonds that contribute substantially to their unusual chemical behavior.