There is a growing interest in uranium coordination chemistry, particularly as it relates to metal–ligand multiple bonding, and in the last decade significant progress has been made in synthesizing oxo, imido, μ-nitrido, and carbene-containing complexes of uranium. This review summarizes the synthesis, structure and reactivity of these complexes, starting from the inception of the field in 1981. Particular attention is focused on the recent developments in this area, such as the synthesis of the bis(imido) analogues of the uranyl ion, and the isolation of the first μ-nitrido complexes of this element.

Metal–ligand multiple bonding in uranium: structure and reactivity

Since uranium has several common paramagnetic oxidation states, i.e., 5f 1 U, 5f 2 U, and 5f 3 U, one of the analytical methods commonly used for characterizing uranium complexes is measurement of magnetic susceptibilities. A variety of solid state and solution methods have been used to obtain magnetic data on uranium complexes under different conditions at both room and low temperature, and data are often cited to support the assignment of oxidation state. Statements like “the magnetic moment of compound X is consistent with U” are common in uranium papers. However, it has been pointed out repeatedly in the literature that evaluating the magnetic susceptibility of uranium complexes is not as straightforward as it is for transition metal and lanthanide complexes. For any metal complex, the magnetic moment arises from the sum of the contributions of the ground state and the low-lying thermally accessible excited states as well as any temperature-independent magnetism in the system. For most 4f lanthanides, the ground state is defined by a LJ Russell−Saunders term because spin−orbit coupling is large and ligand field splitting is small due to the limited radial extension of the 4f orbitals. Roomtemperature magnetic moments can be approximated for most of the lanthanides by μJ = g[J(J + 1)] 1/2 and correlated with oxidation state. This approximation does not apply to Sm and Eu because they have low-lying excited states, but the range of their experimental values is well defined. The situation is more complicated for d transition metals where the ligand field effects are strong and can diminish the contribution of the orbital angular momentum to the magnetic moment (orbital quenching). However, an operational solution exists for transition metal complexes: a spin-only approximation, μS = 2[S(S + 1)] , can be used as a starting point and then modified for orbital contributions depending on the d configuration and symmetry. The actinides are intermediate between lanthanides and transition metals in the sense that the greater radial extension of the 5f orbitals means that ligand field effects cannot be ignored, yet spin−orbit coupling is large. In a case where two effects can be comparable, finding a simple model is usually more difficult. Neither the spin-only μS approximation used for transition metals nor the total angular momentum μJ approximation used for the lanthanides is adequate for uranium, although μJ is often used as an approximate starting point. A detailed theoretical analysis on actinide magnetism has recently been published that defines these issues more formally. A further complication for uranium is that the μJ values of 3.62 and 3.58 μB often used to approximate magnetism for U 3+ and U, respectively, are similar enough to be within the experimental error of typical measurements. It is therefore difficult to differentiate U and U from room-temperature magnetic moments. As a consequence, it is preferable to obtain variable-temperature data with a superconducting quantum interference device (SQUID) or vibrating sample magnetometer (VSM) since the magnetic moment of a 5f 2 U ion typically approaches zero at low temperature due to a singlet ground state. This does not occur with half-integral 5f 3 U and 5f 1 U. Unfortunately, although variable-temperature analysis is the state-of-the-art method to evaluate the magnetism of uranium complexes, such data are not always easily obtained by all research groups and even the shapes of variable-temperature plots are not always consistent for a given ion. Despite these difficulties, many claims of oxidation state based on room-temperature magnetic moments are in the literature. These claims often cite other papers with similar magnetic moments to support the oxidation state assignment without mentioning the papers in which different magnetic moments were observed for that oxidation state. To our knowledge, no comprehensive collection of magnetic data on uranium complexes has been assembled to survey the actual range of data in the literature. To remedy this deficiency, the data in this review were compiled. It was of interest to determine if there were any trends in the ranges of magnetic moments reported for uranium complexes and the frequency of typically observed values. Magnetic data on over 500 complexes of uranium in the common +3, +4, and +5 oxidation states as

Magnetic Susceptibility of Uranium Complexes

The sterically crowded (C5Me5)3U complex reacts with KC8 or K/(18-crown-6) in benzene to form [(C5Me5)2U]2(-6:6-C6H6), 1, and KC5Me5. These reactions suggested that (C5Me5)3U could be susceptible to (C5Me5)1- substitution by benzene anions via ionic salt metathesis. To test this idea in the synthesis of a more conventional product, (C5Me5)3U was treated with KN(SiMe3)2 to form (C5Me5)2U[N(SiMe3)2] and KC5Me5. 1 has long U-C(C5Me5) bond distances comparable to (C5Me5)3U, and it too is susceptible to (C5Me5)1- substitution via ionic metathesis: 1 reacts with KN(SiMe3)2 to make its amide-substituted analogue {[(Me3Si)2N](C5Me5)U}2(-6:6-C6H6), 2. Complexes 1 and 2 have nonplanar C6H6-derived ligands sandwiched between the two uranium ions. 1 and 2 were examined by reactivity studies, electronic absorption spectroscopy, and density functional theory calculations. [(C5Me5)2U]2(-6:6-C6H6) functions as a six-electron reductant in its reaction with 3 equiv of cyclooctatetraene to form [(C5Me5)(C8H8)U]2(-3:3-C8H8), (C5Me5)2, and benzene. This multielectron transformation can be formally attributed to three different sources: two electrons from two U(III) centers, two electrons from sterically induced reduction by two (C5Me5)1- ligands, and two electrons from a bridging (C6H6)2- moiety.

Structure, Reactivity, and Density Functional Theory Analysis of the Six-Electron Reductant, [(C5Me5)2U]2(μ-η6:η6-C6H6), Synthesized via a New Mode of (C5Me5)3M Reactivity

Surface reactions of uranium oxide powder, thin films and single crystals

Terminal Phosphinidene and Heavier Congeneric Complexes. The Quest Is Over

A study of the reaction of the organoactinide hydrides (Cp'/sub 2/MH/sub 2/)/sub 2/ (Cp' = eta/sup 5/-(CH/sub 3/)/sub 5/C/sub 5/, M = Th, U) with trimethyl phosphite is reported. Quantitative transposition of hydride and methoxide ligands occurs to yield the corresponding Cp'/sub 2/M(OCH/sub 3/)/sub 2/ complexes (synthesized independently from Cp'/sub 2/MCl/sub 2/ and NaOCH/sub 3/) and the phosphinidene-bridged methoxy complexes (Cp'/sub 2/M(OCH/sub 3/))/sub 2/PH. The reaction is considerably more rapid for M = U than for M = Th. The new compounds were characterized by elemental analysis, /sup 1/H and /sup 31/P NMR, infrared spectroscopy, magnetic susceptibility, and D/sub 2/O hydrolysis. The molecular structure of (Cp'/sub 2/U(OCH/sub 3/))/sub 2/PH has been determined by single-crystal X-ray diffraction techniques. It crystallizes in the monoclinic space group P2/n with a = 13.926 (3) A, b = 10.765 (3) c = 15.282 (4) A, ..beta.. = 107.63 (2)/sup 0/, and Z = 2. Full-matrix least-squares refinement of the structural parameters for the 24 independent anisotropic non-hydrogen atoms has converged to R/sub 1/ (unweighted, based on F) = 0.041 for 1677 independent absorption-corrected reflections having 2 theta/sub MoK..cap alpha../ 3sigma(I). The (Cp'/sub 2/U(OCH/sub 3/))/sub 2/PH molecule has C/sub 2/more » symmetry, with the ..mu..-PH/sup 2 -/ ligand lying on a crystallographic twofold axis. The coordination geometry about each uranium ion is of the typical pseudotetrahedral Cp'/sub 2/M(X)T type, with U-P = 2.743 (1) A, U-O = 2.046 (14) A, angle U-P-U = 157.7 (2)/sup 0/, and angle U-O-C (methyl) = 178 (1)/sup 0/. Evidence is presented that other > P-OR linkages react in a similar manner.« less

Organoactinide phosphine/phosphite coordination chemistry. Facile hydride-induced dealkoxylation and the formation of actinide phosphinidene complexes

Hydrogenolysis of U((CH/sub 3/)/sub 5/C/sub 5/)/sub 2/R/sub 2/, (R = CH/sub 3/Ch/sub 2/Si(CH/sub 3/)/sub 3/), proceeds at -20/sup 0/C in the presence of excess bis(2 dimethylphosphino)ethane(dmpe) according to this reaction: U((CH/sub 3/)/sub 3/C/sub 5/)/sub 2/R/sub 2/ + 1.5H/sub 2/ + dmpe ..-->.. (toluene, 18h) U((CH/sub 3/)/sub 5/C/sub 5/)/sub 2/(dmpe) H + 2RH. Black microcrystals can be purified by vacuum Soxhlet extraction with toluene. All processes involving this compound must be performed under argon or helium atmospheres, since it reacts with nitrogen. The structure was studied by NMR, ir spectra. The molar magnetic suceptibility was measured at 295 K, 5120 x 10/sup -4/ emu and is consistent with a U (III) formulation. Crystals are orthorhombic. X-ray diffraction data were collected. Structural parameters were refined to convergence. X-ray structural analysis reveals monocrystals of discrete mononuclear U(eta/sup 5/-(CH/sub 3/)/sub 5/C/sub 5/)/sub 2/ (dmpe)H molecules. The molecular structure is evaluated. 1 figure.

Synthesis, properties, and molecular structure of a trivalent organouranium diphosphine hydride

New stoichiometric and catalytic organometallic chemistry with actinides. CH activation and phosphine/phosphite coordination chemistry

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Organoactinide phosphine/phosphite coordination chemistry. Facile hydride-induced dealkoxylation and the formation of actinide phosphinidene complexes

Synthesis, properties, and molecular structure of a trivalent organouranium diphosphine hydride

New stoichiometric and catalytic organometallic chemistry with actinides. CH activation and phosphine/phosphite coordination chemistry