Over 70 years of chemical investigations have shown that plutonium exhibits some of the most complicated chemistry in the periodic table. Six Pu oxidation states have been unambiguously confirmed (0 and +3 to +7), and four different oxidation states can exist simultaneously in solution. We report a new formal oxidation state for plutonium, namely Pu in [K(2.2.2-cryptand)][PuCp″], Cp″ = CH(SiMe). The synthetic precursor PuCp″ is also reported, comprising the first structural characterization of a Pu-C bond. Absorption spectroscopy and DFT calculations indicate that the Pu ion has predominantly a 5f electron configuration with some 6d mixing.

The synthesis of new molecular complexes of U2+ has been pursued to make comparisons in structure, physical properties, and reactivity with the first U2+ complex, [K(2.2.2‐cryptand)][Cp′3U], 1 (Cp′=C5H4SiMe3). Reduction of Cp′′3U [Cp′′=C5H3(SiMe3)2] with KC8 in the presence of 2.2.2‐cryptand or 18‐crown‐6 generates [K(2.2.2‐cryptand)][Cp′′3U], 2‐K(crypt), or [K(18‐crown‐6)(THF)2][Cp′′3U], 2‐K(18c6), respectively. The UV/Vis spectra of 2‐K and 1 are similar, and they are much more intense than those of U3+ analogues. Variable temperature magnetic susceptibility data for 1 and 2‐K(crypt) reveal lower room temperature χMT values relative to the experimental values for the 5f3 U3+ precursors. Stability studies monitored by UV/Vis spectroscopy show that 2‐K(crypt) and 2‐K(18c6) have t1/2 values of 20 and 15 h at room temperature, respectively, vs. 1.5 h for 1. Complex 2‐K(18c6) reacts with H2 or PhSiH3 to form the uranium hydride, [K(18‐crown‐6)(THF)2][Cp′′3UH], 3. Complexes 1 and 2‐K(18c6) both reduce cyclooctatetraene to form uranocene, (C8H8)2U, as well as the U3+ byproducts [K(2.2.2‐cryptand)][Cp′4U], 4, and Cp′′3U, respectively.

Developing a better understanding of covalency (or orbital mixing) is of fundamental importance. Covalency occupies a central role in directing chemical and physical properties for almost any given compound or material. Hence, the concept of covalency has potential to generate broad and substantial scientific advances, ranging from biological applications to condensed matter physics. Given the importance of orbital mixing combined with the difficultly in measuring covalency, estimating or inferring covalency often leads to fiery debate. Consider the 60-year controversy sparked by Seaborg and co-workers ( Diamond, R. M.; Street, K., Jr.; Seaborg, G. T. J. Am. Chem. Soc. 1954 , 76 , 1461 ) when it was proposed that covalency from 5f-orbitals contributed to the unique behavior of americium in chloride matrixes. Herein, we describe the use of ligand K-edge X-ray absorption spectroscopy (XAS) and electronic structure calculations to quantify the extent of covalent bonding in-arguably-one of the most difficult systems to study, the Am-Cl interaction within AmCl. We observed both 5f- and 6d-orbital mixing with the Cl-3p orbitals; however, contributions from the 6d-orbitals were more substantial. Comparisons with the isoelectronic EuCl indicated that the amount of Cl 3p-mixing with Eu 5d-orbitals was similar to that observed with the Am 6d-orbitals. Meanwhile, the results confirmed Seaborg's 1954 hypothesis that Am 5f-orbital covalency was more substantial than 4f-orbital mixing for Eu.

To evaluate the effect of cyclopentadienyl ligand substitution in complexes of new +2 ions of the lanthanides, comparisons in reactivity and spectroscopic and magnetic properties have been made between [K(crypt)][Cp′ 3 Ln], 1-Ln (Cp′ = C 5 H 4 SiMe 3 ; crypt = 2.2.2-cryptand; Ln = La, Ce, Pr, and Nd), and [K(crypt)][Cp′′ 3 Ln], 2-Ln [Cp′′ = C 5 H 3 (SiMe 3 ) 2 ]. The 2-Ln complexes (Ce, Pr, and Nd) were synthesized by reduction of Cp′′ 3 Ln with potassium graphite in the presence of crypt and crystallographically characterized. The structures and UV−visible spectra of 2-Ln are similar to those of 1-Ln, as expected, but greater thermal stability for 2-Ln, expected from comparisons of 2-U and 1-U, was not observed. The magnetic susceptibilities of 2-Ce and 2-Pr were investigated because those of 1-Ce and 1-Pr did not match simple coupling models for 4f n 5d 1 electron configurations. The magnetic data of the 2-Ln complexes are similar to those of 1-Ln, which suggests that Ce 2+ and Pr 2+ complexes with 4f n 5d 1 electron configurations may have more complex electronic structures compared to nontraditional divalent complexes of the later lanthanides. Reactivity studies of isolated samples of 1-Ln and 2-Ln with 1,2-dimethoxyethane (DME) were conducted to determine if methoxide products, found in previous in situ studies of the synthesis of 2-Ln by Lappert and co-workers, would form. Methoxide products were not observed, which shows that the chemistry of the isolated complexes differs from that of the in situ reduction reactions.