Trans effect and inverse trans effect in MLX5 complexes (M=Mo, U; L=O, S; X=Cl, Br): A rationalization within density functional theory study

Observing the Role of Electron Delocalization in Electronic Transport by Incorporating Actinides into Ligated Metal-Chalcogenide Superatoms

Since delocalization of electronic states is a prerequisite for exerting unique electron transport properties, early actinides (An) with highly delocalized 5f/6d orbitals are natural candidates. However, given the experimental difficulties of such radioactive compounds and the complex relativistic effects in theoretical studies, understanding the electronic structure and bonding of actinides is underdeveloped on the periodic table. A further challenge is the very complicated electronic structures encountered in the confinement of actinides, as vividly illustrated by the weakly radioactive Th(Thorium)-encapsulated metal chalcogenide clusters, Th@Co6Te8L6 (L = PH3, PMe3, PEt3). Here we report the electronic structure and the electron transport properties of the Th@Co6Te8L6 clusters and compare them with those of the hollow Co6Te8L6 clusters using the nonequilibrium Green's function combined with relativistic density functional theory (NEGF-DFT). We found that the equilibrium conductance in Th@Co6Te8(PH3)6 (0.76 G0) has been greatly improved over that in Co6Te8(PH3)6 (0.03 G0), which has also been verified under an applied different bias voltage. The covalent bonding character between 6d (Th) and 3d (Co) atomic orbitals resulting from steric confinement is the source of the performance enhancement and a most important factor governing the accessibility of such 5f/6d orbitals. The results are of significance to the rapidly developing field of molecular nanoelectronics.

Solvent-mediated crystallization of (TMS)2BiBr5·DMSO: a new 0D hybrid halide perovskite

Lead-free hybrid halide perovskites have recently gained enormous research attention because of their excellent optical properties. Herein, we report a novel zero-dimensional (0D) hybrid halide perovskite, (TMS)₂BiBr₅·DMSO, stabilized by the dimethyl sulfoxide (DMSO) solvent. The structure contains isolated [BiBr₅OS(CH₃)₂]^2- anionic polyhedra and the cations [(CH₃)₃S]⁺ (trimethyl sulfonium ion, TMS) balance the charge, making it a 0D halide perovskite. Non-interaction of the trans Br atoms (with respect to the Bi-DMSO bond) with the cationic TMS resulted in shorter trans Bi-Br bonds as compared to other Bi-Br bonds in the isolated BiBr₅·DMSO polyhedra. The optical properties study reveals that the compound is an indirect band gap type with a band gap of 2.79 eV. From the PL study it is observed that the compound shows emission in the blue region.

cis- and trans-Ligand Effects on the Inverse trans-Influence in [UVI(O)(L)Cl4]0/- (L = Unidentate Ligand) Complexes

A comprehensive exploration of the inverse trans-influence (ITI) phenomenon in a series of cis-[U^VI(O)(L)Cl₄]^0/- and trans-[U^VI(O)(L)Cl₄]^0/- complexes involving a wide variety of neutral and anionic unidentate ligands L, using relativistic density functional theory, threw light on the still-intriguing physics of ITI, elucidated its origin, and deployed the ligands L in cis- and trans-ITI sequences (ladders). ITI is produced for the complete set of L in both series of [U(O)(L)Cl₄]^0/- complexes, but this is not reflected in the thermodynamic stability of the [U(O)(L)Cl₄]^0/- isomers. In effect the hard and strong σ-donor anionic ligands stabilize the trans isomers, but the opposite is true for the soft σ-donor/π-donor neutral and anionic ligands that stabilize the cis isomers. According to the ITI%(U-L) metrics the hard strong σ-donor anionic ligands exert stronger ITI than the soft σ-donor/π-donor neutral ones, while according to the ITI%(U-O) metrics ITI is produced only for the more stable trans-[U(O)(L)Cl₄]^0/- isomers involving the anionic ligands. In contrast the neutral ligands in the more stable cis-[U(O)(L)Cl₄]^0/- isomers produce the normal cis influence (CI). Furthermore, the more electronegative ligands produce stronger ITI. ITI%(U-O) cis- and trans-philicity ladders are also built for both series of complexes employing the isotropic σ^iso(SO) ¹⁷O NMR shielding constants as a sensitive metric of the ITI phenomenon. The NMR ITI%(U-O) metrics are consistent with the ITI%(U-O) ones illustrating that the isotropic ¹⁷O NMR shifts are sensitive metrics of the covalency of the multiple U-O bonding mode and, hence, of the ITI phenomenon. Interestingly the 2σ BD(U-O) natural bond orbitals play a key role in tuning the bond length and covalency of the U-O bond through the 2σ(U≡O) → 2σ*(U≡O) hyperconjugative interactions. The assessment of the magnitude of the ITI in the [U^VI(O)(L)Cl₄]^0/- complexes and the recognition of the factors affecting ITI dispose a guide to experimentalists working in the area of uranium chemistry to develop strategies for stabilizing uranium-ligand linkages.

Emergence of the structure-directing role of f-orbital overlap-driven covalency

FEUDAL (f’s essentially unaffected, d’s accommodate ligands) is a longstanding bonding model in actinide chemistry, in which metal-ligand binding uses 6d-orbitals, with the 5f remaining non-bonding. The inverse-trans-influence (ITI) is a case where the model may break down, and it has been suggested that ionic and covalent effects work synergistically in the ITI. Here, we report an experimentally grounded computational study that quantitatively explores the ITI, and in particular the structure-directing role of f-orbital covalency. Strong donor ligands generate a cis-ligand-directing electrostatic potential (ESP) at the metal centre. When f-orbital participation, via overlap-driven covalency, becomes dominant via short actinide-element distances, this ionic ESP effect is overcome, favouring a trans-ligand-directed geometry. This study contradicts the accepted ITI paradigm in that here ionic and covalent effects work against each other, and suggests a clearly non-FEUDAL, structure-directing role for the f-orbitals.

In actinide chemistry, a longstanding bonding model describes metal-ligand binding using 6d-orbitals, with the 5f-orbitals remaining non-bonding. Here the authors explore the inverse-trans-influence — a case where the model breaks down — finding that the f-orbitals play a crucial role in dictating a trans-ligand-directed geometry.

Rare-earth metal and actinide organoimide chemistry

Elaborate synthesis schemes pave the way to f-element and group 3 complexes with multiply bonded imido ligands displaying intriguing reactivity.

Towards the first theoretical scale of the trans effect in octahedral complexes

In this paper, we show that trans effects in octahedral complexes can primarily be related to differences in the ability, for a given ligand, to cede electron density to the metal cation under the influence of the ligand at the trans position.

Uranium-ligand multiple bonding in uranyl analogues, [L═U═L]n+, and the inverse trans influence

The societal importance of uranium complexes containing the uranyl moiety [O═U═O](2+) continues to grow with the ongoing international nuclear enterprise and associated accumulating legacy waste. Further studies of the electronic structure of uranyl and its analogues are imperative for the development of crucial technologies, including lanthanide/actinide extractants and chemical and environmental remediation methodologies. Actinide oxo halides are a subset of the growing class of actinyl (uranyl) analogues. The understanding of their electronic structures links the detailed spectroscopic studies of uranyl, indicating the role of the pseudocore 6p orbitals in U-O bonding, to hypotheses about the 6p orbitals' role in the chemical bonding of uranyl analogues. These actinide oxo halides are a very small class of actinide compounds that present the inverse trans influence (ITI). This class of complexes was, until recently, limited to two crystallographically characterized compounds, namely, [UCl(5)O][PPh(4)] and [PaCl(5)O][NEt(4)](2). These complexes are important because they give a readily and clearly defined experimental observable: the difference between the M-X(trans) and M-X(cis) (here X = Cl) bond lengths in the solid state. This bond metric is a sensitive probe for the role of 6p, 6d, and 5f orbitals, as well as electrostatic interactions, in determining their electronic structure. This Viewpoint Article reviews the theoretical, experimental, and synthetic work on the ITI in actinide complexes and contextualizes it within broader studies on the electronic structure of uranyl and its analogues. Furthermore, our recent work on the ITI in high-valent uranium(V/VI) oxo and imido complexes is described as a whole. This work builds on the extant synthetic literature on the ITI and provides design parameters for the synthesis and characterization of high-valent uranium-ligand multiple bonds.

Observation of the inverse trans influence (ITI) in a uranium(V) imide coordination complex: an experimental study and theoretical evaluation

An inverse trans influence has been observed in a high-valent U(V) imide complex, [(((Ad)ArO)(3)N)U(NMes)]. A thorough theoretical evaluation has been employed in order to corroborate the ITI in this unusual complex. Computations on the target complex, [(((Ad)ArO)(3)N)U(NMes)], and the model complexes [(((Me)ArO)(3)N)U(NMes)] and [(NMe(3))(OMe(2))(OMe)(3)U(NPh)] are discussed along with synthetic details and supporting spectroscopic data. Additionally, the syntheses and full characterization data of the related U(V) trimethylsilylimide complex [(((Ad)ArO)(3)N)U(NTMS)] and U(IV) azide complex [(((Ad)ArO)(3)N)U(N(3))] are also presented for comparison.

Synthesis of uranium(VI) terminal oxo complexes: molecular geometry driven by the inverse trans-influence

Oxidation of our previously reported uranium(V) oxo complexes, supported by the chelating ((R)ArO)(3)tacn(3-) ligand system (R = tert-butyl (t-Bu), 1-t-Bu; R = 1-adamantyl (Ad), 1-Ad), yields terminal uranium(VI) oxo complexes [(((R)ArO)(3)tacn)U(VI)(O)]SbF(6) (R = t-Bu, 2-t-Bu; R = Ad, 2-Ad). These complexes differ in their molecular geometry in that 2-t-Bu possesses pseudo-C(s) symmetry in solution and solid state as the terminal oxo ligand lies in the equatorial plane (as defined by the three aryloxide arms of the ligand) in order to accommodate the thermodynamic preference of high-valent uranium oxo complexes to have a σ- and π-donating ligand trans to the oxo (vis-à-vis the ubiquity of the linear UO(2)(2+) moiety). The distortion of the ligand--which stands in contrast to all other complexes of uranium supported by the ((R)ArO)(3)tacn(3-) ligand, including 2-Ad--is most clearly seen in the structures of 2-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)]SbF(6), and 3-t-Bu, [(((t-Bu)ArO)(3)tacn)U(VI)(O)(eq)(OC(O)CF(3))(ax)]. The solid-state structure of 3-t-Bu reveals that the trans U-O(ArO) bond length is shortened by 0.1 Å in comparison to the cis U-O(ArO) bonds and the trans U-O-C(ipso) angle is linearized (157.67° versus 147.85° and 130.03°). Remarkably, the minor modification of the ligand to have Ad groups at the ortho positions of the aryloxide arms is sufficient to stabilize a C(3v)-symmetric terminal uranium(VI) oxo complex (2-Ad) without a ligand trans to the oxo. These experimental results were reproduced in DFT calculations and allow the qualitative bracketing of the relative thermodynamic stabilization afforded by the inverse trans-influence as ∼6 kcal mol(-1).