Electronic, Magnetic, and Theoretical Characterization of (NH₄)₄UF₈, a Simple Molecular Uranium(IV) Fluoride

Abstract

The simple system of tetraammonium octafluorouranate is employed to derive a fundamental understanding of the uranium–fluorine interaction. The structure is composed of isolated molecules, enabling a detailed examination of the U4+ (f 2) ion. Characterization of single-crystals by X-ray diffraction, absorption spectroscopy, and magnetic analysis up to 45 T is combined with extensive theoretical treatment by CASSCF. The influence of different active spaces and representations of the structure is examined in the context of the experimental evidence. The Interacting Quantum Atoms method (IQA) is used to examine the nature of the U–F bond, concluding that there is a non-negligible degree of covalent character (9% of the total bond energy) in [UF8]4−. For the structural and theoretical reasons discussed herein, it is proposed that the structure of (NH4)4UF8 may be appropriately employed as a benchmark compound for future theoretical characterization of U­(IV).

Probing a variation of the inverse-trans-influence in americium and lanthanide tribromide tris(tricyclohexylphosphine oxide) complexes

“…Key elements within QTAIM were extracted such as the electron density, delocalization indices, and energy densities at the interatomic region (bond critical point, BCP) which have been employed previously for this aim. [40][41][42][43][44][45][46][47][48] The covalency was analyzed, on one hand, by changes in the concentration of the electron density at the BCP along with changes in the delocalization indices. On the other hand, energy densities show the polarization of the covalent bond by looking at the ratios between potential [V(r)] and kinetic [G(r)] energy densities, which for partial covalent bonds lie in between values of 1 and 2.…”

“…Based on previous studies where the nature of bonding interactions of fblock complexes has been shown that bonds are not formally covalent, but rather partially covalent. 46,47,93 A partial covalent bond implies positive values for the Laplacian of the electron density, Vr(r), and negative values for total energy densities, H(r). Here, the term "covalency" refers to the metal-ligand interaction needed to experience orbital overlap, which is re-ected by the buildup of electron density [r(r)] in the interatomic region.…”

A Molecular Octafluoridoneptunate(IV) Anion in (NH4)4[NpF8] and Theoretical Investigations of the [MF8]4– System (M = Th – Bk)

“…[3] In recent years single-crystal structure determinations of ternary fluoridometallates(IV) have mainly been carried out for Th and U compounds, whereas such studies and thus detailed structural parameters for compounds of the transuranium elements are lacking. [3][4][5][6][7] For the latter, usually only data obtained from powder diffraction studies have been reported, with the exception of a few single-crystal structure determinations. [1,2,[8][9][10][11][12][13] The coordination number of the actinide atom in ternary fluoridometallates(IV) is usually nine, with a tricapped-trigonal prismatic arrangement of the fluorido ligands.…”

“…[1,2,[8][9][10][11][12][13] The coordination number of the actinide atom in ternary fluoridometallates(IV) is usually nine, with a tricapped-trigonal prismatic arrangement of the fluorido ligands. [3,4] Compounds with a coordination number of eight of the actinide atom are scarce for fluoridometallates(IV), examples being Li 4 [ [6,11,[14][15][16] In these compounds the complex anions are spatially separated and not connected to each other. For coordination number eight commonly four different ligand arrangements are observed -the ideal polyhedra being either a cube (O h ), a square antiprism (D 4d ), a snub disphenoid (D 2d , Johnson solid J 84 ) or a bicapped trigonal prism (C 2v ).…”

“…[33][34][35][36] Border cases are (NH 4 ) 4 [UF 8 ] and K 5 ThF 9 , in which the [MF 8 ] 4anions are distorted and could either be classified as a squareantiprism or a bicapped trigonal prism. [6,15,16] This is probably also the case for (NH 4 ) 4 [MF 8 ] (M = Ce, Pa, Np, Pu) but only powder diffraction or EXAFS (Np) data has been reported so far and the single-crystal structures have not yet been deter-mined. [37][38][39][40] In comparison to (NH 4 ) 4 [UF 8 ], the Th compound (NH 4 ) 4 ThF 8 does not show molecular anions, but a more complex anion with the Niggli formula 1 ∞ [ThF 4/2 F 5/1 ] 3-.…”