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Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA) 3 ( 2 ) with KC 8 yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA) 2 ] ( 3 ) (TDA = N -(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f 4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I 2 to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA) 2 ) 2 (μ-I)] ( 6 ) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η 7 -C 7 H 7 )(TDA) 2 (THF)] ( 7 ). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA) 4 ] ( 1-crypt ) and U(COT) 2 , while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA) 3 ] 2 (μ-η 8 :η 3 -C 8 H 8 ) ( 8 ). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA) 4 ( 4 ) with KC 8 gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA) 3 (THF)] ( 5 ). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

Electron paramagnetic resonance (EPR) studies of the rhenium(II) complex Re(η 5 -Cp)(BDI) ( 1 ; BDI = N , N '-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) have revealed that this species reversibly binds N 2 in solution: flash frozen toluene solutions of 1 disclose entirely different EPR spectra at 10 K when prepared under N 2 versus Ar atmospheres. This observation was additionally verified by the synthesis of stable CO and 2,6-xylylisocyanide (XylNC) adducts of 1 , which display EPR features akin to those observed in the putative N 2 complex. While we found that 1 displays an extremely large g max value of 3.99, the binding of an additional ligand leads to substantial decreases in this value, displaying g max values of ca. 2.4. Following the generation of isotopically enriched 15 N 2 and 13 CO adducts of 1 , HYSCORE experiments allowed for the measurement of the corresponding hyperfine couplings associated with spin delocalization onto the electron-accepting ligands in these species, which proved to be small. A cumulative assessment of the EPR data, when combined with insights provided by near-infrared (NIR) spectroscopy and time-dependent density functional theory (TDDFT) calculations, indicated that while the binding of electron acceptors to 1 does lead to decreases in g max in relative accord with the field strength (i.e., π-acidity) of the variable ligand, the magnitude of these decreases is primarily due to the changes in electronic structure at the Re center.