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• Metal-ligand bonding in lanthanide(III) complexes is ionic in nature • Quantitative calculations reveal that metal-ligand bonding in lanthanide-metallocene complexes has a very significant covalent contribution • Metal-ligand covalency in lanthanide metallocene complexes involves the vacant lanthanide 6 s , 5 d and 6 p shells [Display omitted] Metal-ligand bonding in bis(cyclopentadienyl) and bis(cyclobutadienyl) lutetium(iii) complexes was studied in a variety of geometries using density functional theory. Based on analysis of the orbitals, the Morokuma–Ziegler–Rauk extended transition state method and natural orbitals of the chemical valence, the bonding interaction is dominantly ionic with a very important covalent contribution. The relative contribution of covalency is so large that it should not be neglected even in qualitative treatments of the electronic structure. The covalency arises from electron donation from the ligand HOMOs and HOMO–1 orbitals to the vacant 5 d , 6 s and 6 p orbitals at the Lu 3 + ion. While 5 d and the 6 s orbitals play the most important part, the 6 p orbitals also make a comparable contribution. A similar bonding picture emerges both in realistic low-symmetry metallocenes and idealized high-symmetry geometries. The results put to question the common view of the metal–ligand bonding in lanthanide complexes as completely or almost completely of ionic character, and have implications in the design of crystal-fields in lanthanide complexes when targeting specific magnetic properties. [ABSTRACT FROM AUTHOR]

Despite there being numerous examples of f‐element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5–8), cyclobutadienyl (C4) complexes remain exceedingly rare. Here, we report that reaction of [Li2{C4(SiMe3)4}(THF)2] (1) with [U(BH4)3(THF)2] (2) gives the pianostool complex [U{C4(SiMe3)4}(BH4)3][Li(THF)4] (3), where use of a borohydride and preformed C4‐unit circumvents difficulties in product isolation and closing a C4‐ring at uranium. Complex 3 is an unprecedented example of an f‐element half‐sandwich cyclobutadienyl complex, and it is only the second example of an actinide‐cyclobutadienyl complex, the other being an inverse‐sandwich. The U−C distances are short (av. 2.513 Å), reflecting the formal 2− charge of the C4‐unit, and the SiMe3 groups are displaced from the C4‐plane, which we propose maximises U−C4 orbital overlap. DFT calculations identify two quasi‐degenerate U−C4 π‐bonds utilising the ψ2 and ψ3 molecular orbitals of the C4‐unit, but the potential δ‐bond using the ψ4 orbital is vacant. [ABSTRACT FROM AUTHOR]

The chemistry of arene complexes of uranium has for over half a century been an important facet of organoactinide chemistry. Within this extensive and burgeoning field, in the past two decades inverted sandwich complexes have emerged incorporating cyclobutadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands. Herein, the field is reviewed with an emphasis on well-defined molecular species that have been unambiguously characterised by X-ray crystallography. [ABSTRACT FROM AUTHOR]