δ and φ back-donation in AnIV metallacycles

• Published in: Nature communications Vol. 11; no. 1; p. 1558
• Main Authors: Kelley, Morgan P., Popov, Ivan A., Jung, Julie, Batista, Enrique R., Yang, Ping
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25.03.2020; Nature Publishing Group; Nature Portfolio
• Subjects: 119/118; 639/638/263/406; 639/638/263/910; 639/638/563/606; Actinide metal compounds; Actinides; Chemical behavior; Contraction; Donations; Humanities and Social Sciences; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ligands; multidisciplinary; Orbitals; Organometallic compounds; Plutonium; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-15197-w

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. Metal-ligand δ and φ 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 δ and φ back-bonds that contribute substantially to their unusual chemical behavior.