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Abstract For the first time, the capture of the planar antiaromatic parent benzene dianion in between two trivalent rare earth (RE) metal cations (RE^III), each stabilized by two guanidinate ligands, is reported. The synthesized inverse‐sandwich complexes [{(Me₃Si)₂NC(NⁱPr)₂}₂RE]₂(μ‐η⁶ : η⁶‐C₆H₆), (RE=Y (1), Dy (2), and Er (3)) were crystallized from aprotic solvents and feature a remarkably planar parent benzene dianion, previously not encountered for any metal ion prone to low or absent covalency. The −2 charge localization at the benzene ligand was deduced from the results obtained by single‐crystal X‐ray diffraction analyses, spectroscopy, magnetometry, and Density Functional Theory (DFT) calculations. In the¹H NMR spectrum of the diamagnetic Y complex1, the equivalent proton resonance of the bridging benzene dianion ligand is drastically shifted to higher field in comparison to free benzene. This and the calculated highly positive Nucleus‐Independent Chemical Shift (NICS) values are attributed to the antiaromatic character of the benzene dianion ligand. The crucial role of the ancillary guanidinate ligand scaffold in stabilizing the planar benzene dianion conformation was also elucidated by DFT calculations. Remarkably, the planarity of the benzene dianion originates from the stabilization of the π‐type orbitals of the d‐manifold and compression through its strong electrostatic interaction with the two RE^IIIsites. 

Abstract Anionic ancillary ligands play a critical role in the construction of rare earth (RE) metal complexes due to the large influence on the stability of the molecule and engendering emergent electronic properties that are of interest in a plethora of applications. Supporting ligands comprising oxygen donor atoms are highly pursued in RE chemistry owing to the high oxophilicity innate to these ions. The scarcely employed bis(acyl)phosphide (BAP) ligands feature oxygen coordination sites and contain a phosphide backbone rendering it attractive for RE‐coordination chemistry. Here, we integrate bis(mesitoyl)phosphide (^mesBAP) as an ancillary ligand into RE^IIIchemistry to generate the first dinuclear trivalent RE complexes containing BAP ligands; [{^mesBAP}₂RE(THF)(μ‐Cl)]₂(RE=Y, (1), Gd (2), and Dy (3); THF=tetrahydrofuran). Each RE center is ligated to two monoanionic^mesBAP ligands, one THF molecule and one chloride ion. All three molecules were characterized through single‐crystal X‐ray diffraction,³¹P NMR, IR and UV‐Vis spectroscopy.³¹P,¹H and¹³C NMR on the diamagnetic yttrium congener1confirm asymmetric ligand coordination. DFT calculations conducted on2provided insight into the electronic structure. The magnetic properties of2and3were investigated via SQUID magnetometry. The Gd^IIIions exhibit weak antiferromagnetic coupling, corroborated by DFT results. 

The π-expanded dibenzocyclooctatetraene ligand was employed to access an unprecedented series of actinide sandwich compounds. The trivalent uranium complexes were studied by crystallography, spectroscopy, and computations. 

An unprecedented dinuclear rare earth metal complex with a bridging 2,2'-azobispyridyl radical anion was isolated from the one-electron oxidation of the parent neutral complex. Both were characterized through crystallography, spectroscopy, and DFT. 

The first complexes containing a 2,3,4,5-tetraiodopyrrolyl anion as a ligand were isolated. The ligands' tendency to foster intermolecular σ-hole interactions was probed and compared to rare earth metal complexes bearing a 2,5-dimethylpyrrolyl anion.