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Abstract Lanthanides in the trivalent oxidation state are typically described using an ionic picture that leads to localized magnetic moments. The hierarchical energy scales associated with trivalent lanthanides produce desirable properties for e.g., molecular magnetism, quantum materials, and quantum transduction. Here, we show that this traditional ionic paradigm breaks down for praseodymium in the tetravalent oxidation state. Synthetic, spectroscopic, and theoretical tools deployed on several solid-state Pr 4+ -oxides uncover the unusual participation of 4 f orbitals in bonding and the anomalous hybridization of the 4 f 1 configuration with ligand valence electrons, analogous to transition metals. The competition between crystal-field and spin-orbit-coupling interactions fundamentally transforms the spin-orbital magnetism of Pr 4+ , which departs from the J eff  = 1/2 limit and resembles that of high-valent actinides. Our results show that Pr 4+ ions are in a class on their own, where the hierarchy of single-ion energy scales can be tailored to explore new correlated phenomena in quantum materials. 

Abstract Actinide diatomic molecules are ideal models to study elusive actinide multiple bonds, but most of these diatomic molecules have so far only been studied in solid inert gas matrices. Herein, we report a charged U≡N diatomic species captured in fullerene cages and stabilized by the U-fullerene coordination interaction. Two diatomic clusterfullerenes, viz. UN@C_s(6)-C₈₂and UN@C₂(5)-C₈₂, were successfully synthesized and characterized. Crystallographic analysis reveals U-N bond lengths of 1.760(7) and 1.760(20) Å in UN@C_s(6)-C₈₂and UN@C₂(5)-C₈₂. Moreover, U≡N was found to be immobilized and coordinated to the fullerene cages at 100 K but it rotates inside the cage at 273 K. Quantum-chemical calculations show a (UN)²⁺@(C₈₂)²⁻electronic structure with formal +5 oxidation state (f¹) of U and unambiguously demonstrate the presence of a U≡N bond in the clusterfullerenes. This study constitutes an approach to stabilize fundamentally important actinide multiply bonded species. 

Abstract The synthesis of bona fide organometallic Ce IV complexes is a formidable challenge given the typically oxidizing properties of the Ce IV cation and reducing tendencies of carbanions. Herein, we report a pair of compounds comprising a Ce IV  − C aryl bond [Li(THF) 4 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-THF ) and [Li(DME) 3 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-DME ), ortho -oxa = dihydro-dimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide, MBP 2–  = 2,2′-methylenebis(6- tert -butyl-4-methylphenolate), which exhibit Ce IV  − C aryl bond lengths of 2.571(7) – 2.5806(19) Å and strongly-deshielded, Ce IV  − C ipso 13 C{ 1 H} NMR resonances at 255.6 ppm. Computational analyses reveal the Ce contribution to the Ce IV  − C aryl bond of 3-THF is ~12%, indicating appreciable metal-ligand covalency. Computations also reproduce the characteristic 13 C{ 1 H} resonance, and show a strong influence from spin-orbit coupling (SOC) effects on the chemical shift. The results demonstrate that SOC-driven deshielding is present for Ce IV  − C ipso 13 C{ 1 H} resonances and not just for diamagnetic actinide compounds. 

Abstract Homoleptic σ‐bonded uranium–alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium–methyl complexes. Amongst these complexes, the first example of a homoleptic uranium–alkyl dimer, [Li(THF)₄]₂[U₂(CH₃)₁₀], as well as a seven‐coordinate uranium–methyl monomer, {Li(OEt₂)Li(OEt₂)₂UMe₇Li}_nwere both crystallographically identified. The diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium–alkyl chemistry. 

Abstract The synthesis and characterization of sterically unencumbered homoleptic organouranium aryl complexes containing U−C σ‐bonds has been of interest to the chemical community for over 70 years. Reported herein are the first structurally characterized, sterically unencumbered homoleptic uranium (IV) aryl‐ate species of the form [U(Ar)₆]²⁻(Ar=Ph,p‐tolyl,p‐Cl‐Ph). Magnetic circular dichroism (MCD) spectroscopy and computational studies provide insight into electronic structure and bonding interactions in the U−C σ‐bond across this series of complexes. Overall, these studies solve a decades‐long challenge in synthetic uranium chemistry, enabling new insight into electronic structure and bonding in organouranium complexes.