Photoelectron spectroscopy of f-element organometallic complexes. 6. Electronic structure of tetrakis(cyclopentadienyl)actinide complexes

Organometallic Neptunium Chemistry

Fifty years have passed since the foundation of organometallic neptunium chemistry, and yet only a handful of complexes have been reported, and even fewer have been fully characterized. Yet, increasingly, combined synthetic/spectroscopic/computational studies are demonstrating how covalently bonding, soft, carbocyclic organometallic ligands provide an excellent platform for advancing the fundamental understanding of the differences in orbital contributions and covalency in f-block metal-ligand bonding. Understanding the subtleties is the key to the safe handling and separations of the highly radioactive nuclei. This review describes the complexes that have been synthesized to date and presents a critical assessment of the successes and difficulties in their analysis and the bonding information they have provided. Because of increasing recent efforts to start new Np-capable air-sensitive inorganic chemistry laboratories, the importance of radioactivity, the basics of Np decay and its ramifications (including the radiochemical synthesis of one organometallic compound), and the available anhydrous starting materials are also surveyed. The review also highlights a range of instances in which important differences in the chemical behavior between Np and its closest neighbors, uranium and plutonium, are found.

Expanding the Chemistry of Molecular U(2+) Complexes: Synthesis, Characterization, and Reactivity of the {[C5 H3 (SiMe3 )2 ]3 U}(-) Anion

The synthesis of new molecular complexes of U(2+) has been pursued to make comparisons in structure, physical properties, and reactivity with the first U(2+) complex, [K(2.2.2-cryptand)][Cp'3 U], 1 (Cp'=C5 H4 SiMe3 ). Reduction of Cp''3 U [Cp''=C5 H3 (SiMe3 )2 ] with KC8 in the presence of 2.2.2-cryptand or 18-crown-6 generates [K(2.2.2-cryptand)][Cp''3 U], 2-K(crypt), or [K(18-crown-6)(THF)2 ][Cp''3 U], 2-K(18c6), respectively. The UV/Vis spectra of 2-K and 1 are similar, and they are much more intense than those of U(3+) analogues. Variable temperature magnetic susceptibility data for 1 and 2-K(crypt) reveal lower room temperature χM T values relative to the experimental values for the 5f(3) U(3+) 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''3 UH], 3. Complexes 1 and 2-K(18c6) both reduce cyclooctatetraene to form uranocene, (C8 H8 )2 U, as well as the U(3+) byproducts [K(2.2.2-cryptand)][Cp'4 U], 4, and Cp''3 U, respectively.

Does covalency increase or decrease across the actinide series? Implications for minor actinide partitioning

A covalent chemical bond carries the connotation of overlap of atomic orbitals between bonded atoms, leading to a buildup of the electron density in the internuclear region. Stabilization of the valence 5f orbitals as the actinide series is crossed leads, in compounds of the minor actinides americium and curium, to their becoming approximately degenerate with the highest occupied ligand levels and hence to the unusual situation in which the resultant valence molecular orbitals have significant contributions from both actinide and the ligand yet in which there is little atomic orbital overlap. In such cases, the traditional quantum-chemical tools for assessing the covalency, e.g., population analysis and spin densities, predict significant metal-ligand covalency, although whether this orbital mixing is really covalency in the generally accepted chemical view is an interesting question. This review discusses our recent analyses of the bonding in AnCp3 and AnCp4 (An = Th-Cm; Cp = η(5)-C5H5) using both the traditional tools and also topological analysis of the electron density via the quantum theory of atoms-in-molecules. I will show that the two approaches yield rather different conclusions and suggest that care must be taken when using quantum chemistry to assess metal-ligand covalency in this part of the periodic table. The implications of this work for minor actinide partitioning from nuclear wastes are discussed; minor actinide extractant ligands based on nitrogen donors have received much attention in recent years, as have comparisons of the extent of covalency in actinide-nitrogen bonding with that in analogous lanthanide systems via quantum-chemical studies employing the traditional tools for assessing the covalency.

Covalency in AnCp4 (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses

The geometric and electronic structures of the title compounds are calculated with scalar relativistic, gradient-corrected density functional theory. The most stable geometry of ThCp(4) (Cp = eta(5)-C(5)H(5)) and UCp(4) is found to be pseudo-tetrahedral (S(4)), in agreement with experiment, and all the other AnCp(4) compounds have been studied in this point group. The metal-Cp centroid distances shorten by 0.06 A from ThCp(4) to NpCp(4), in accord with the actinide contraction, but lengthen again from PuCp(4) to CmCp(4). Examination of the valence molecular orbital structures reveals that the highest-lying Cp pi(2,3)-based orbitals split into three groups of pseudo-e, t(2) and t(1) symmetry. Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series, such that in CmCp(4), the 5f manifold is at more negative energy than the Cp pi(2,3)-based levels. The stability of the Cm 5f orbitals leads to an intramolecular ligand-->metal charge transfer, generating a Cm(III) f(7) centre and increased Cm-Cp centroid distance. Mulliken population analysis shows metal d orbital participation in the e and t(2) Cp pi(2,3)-based orbitals, which gradually decreases across the actinide series. By contrast, metal 5f character is found in the t(1) levels, and this contribution increases four-fold from ThCp(4) to AmCp(4). Examination of the t(1) orbitals suggests that this f orbital involvement arises from a coincidental energy match of metal and ligand orbitals, and does not reflect genuinely increased covalency (in the sense of appreciable overlap between metal and ligand levels). Atoms-in-molecules analysis of the electron densities of the title compounds (together with a series of reference compounds: C(2)H(6), C(2)H(4), Cp(-), M(CO)(6) (M = Cr, Mo, W), AnF(3)CO (An = U, Am), FeCp(2), LaCp(3), LaCl(3) and AnCl(4) (An = Th, Cm)) indicates that the An-Cp bonding is very ionic, increasingly so as the actinide becomes heavier. Caution is urged when using early actinide/lanthanide comparisons as models for minor actinides/middle lanthanides.

Photoinduced Ligand Redistribution Chemistry of Quadruply Bonded Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) Complexes

The quadruply bonded metal-metal complexes cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) (R(3) = Et(3), Me(3), Me(2)Ph, MePh(2); 6-mhp = 2-hydroxy-6-methylpyridinato) photoreact when their solutions are irradiated with visible and near-UV light. The primary photoprocess leads to the ligand redistribution products Mo(2)Cl(3)(6-mhp)(PR(3))(3) and Mo(2)Cl(6-mhp)(3)(PR(3)). In THF at room temperature, these photoproducts are stable and over time they back-react completely to the starting material. Photolysis of cis-Mo(2)Cl(2)(6-mhp)(2)(PR(3))(2) in DMF results in the same products; however, Mo(2)Cl(3)(6-mhp)(PR(3))(3) rapidly decomposes, leaving Mo(2)Cl(6-mhp)(3)(PR(3)) as the only isolable photoproduct. Conversely, when the reaction is carried out in benzene, Mo(2)Cl(6-mhp)(3)(PR(3)) undergoes a slow secondary photoreaction and Mo(2)Cl(3)(6-mhp)(PR(3))(3) is the photoproduct that is isolated. At a given wavelength, the photolysis quantum yield (Phi(p)) increases along the solvent series C(6)H(6) < THF < DMF (Phi(p)(405) = 0.00042, 0.00064, and 0.00097, respectively, for cis-Mo(2)Cl(2)(6-mhp)(2)(PMe(2)Ph)(2)). For a given solvent, Phi(p) increases with decreasing excitation wavelength (Phi(p)(546) = 0.00012, Phi(p)(436) = 0.00035, Phi(p)(405) = 0.00042, Phi(p)(366) = 0.0022, and Phi(p)(313) = 0.0079 in C(6)H(6)). This wavelength dependence of the photoreaction quantum yield in conjunction with the excitation spectrum establishes that the photoreaction does not originate from the lowest energy deltadelta excited state, which possesses a long lifetime and an appreciable emission quantum yield in C(6)H(6), CH(2)Cl(2), THF, and DMF. The photochemistry is instead derived from higher energy excited states with the maximum photoreactivity observed for excitation wavelengths coinciding with absorption features previously assigned to ligand-to-metal charge transfer transitions.