Metal-based Orbitals Research Articles

Rare Earth Metal Complexes Supported by a Tripodal Tris(amido) Ligand System Featuring an Arene Anchor

A new tripodal tris(amido) ligand system featuring an arene anchor was developed and applied to the coordination chemistry of rare earth metals. Two tris(amido) ligands with a 1,3,5-triphenylbenzene backbone were prepared in two steps from commercially available reagents on a gram scale. Salt metathesis and alkane elimination reactions were exploited to prepare mononuclear rare earth metal complexes in moderate to good yields. For salt metathesis reactions, while metal tribromides yielded neutral metal tris(amido) complexes, metal trichlorides led to the formation of ate complexes with an additional chloride bound to the metal center. The new compounds were characterized by X-ray crystallography, elemental analysis, and 1H and 13C nuclear magnetic resonance spectroscopy. The rare earth metal complexes exhibit a trigonal planar coordination geometry for the [MN3] fragment in the solid state rather than a trigonal pyramidal geometry, commonly observed for rare earth metal tris(amido) complexes such as M[N(SiMe3)2]3. Moreover, the arene anchor of the tripodal ligands is engaged in a nonnegligible interaction with the rare earth metal ions. Density functional theory calculations were performed to gain insight into the bonding interactions between the tripodal ligands and the rare earth metal ions. While LUMOs of these rare earth metal complexes are mainly π* orbitals of the arene with a minor component of metal-based orbitals, HOMO-15 and HOMO-16 of a lanthanum complex show that the arene anchor serves as a π donor to the trivalent lanthanum ion.

Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis.

Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)3 groups attached to the CN– ligands stabilize the metal-based t2g-like orbitals by ∼0.8 eV, leading to high 3MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the respective isocyanoborato complexes amenable to photochemical reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C–C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compounds to current challenges in synthetic photochemistry and solar energy conversion.

Comparison of multireference ab initio wavefunction methodologies for X-ray absorption edges: A case study on [Fe(II/III)Cl4]2-/1- molecules.

In this work, we present a detailed comparison of wavefunction-based multireference (MR) techniques for the prediction of transition metal L-edge X-ray absorption spectroscopy (XAS) using [Fe(II)Cl4]2- and [Fe(III)Cl4]1- complexes as prototypical test cases. We focus on the comparison of MR Configuration Interaction (MRCI) and MR Equation of Motion Coupled Cluster (MREOM-CC) methods, which are employed to calculate valence excitation as well as core to valence Fe L-edge XAS spectra of [Fe(II)Cl4]2- and [Fe(III)Cl4]1- complexes. The two investigated approaches are thoroughly analyzed with respect to their information content regarding (1) metal-ligand covalency, (2) ligand field splittings, (3) relativistic effects, (4) electron correlation, (5) energy distribution, and (6) intensity modulation of the experimentally observed spectral features. It is shown that at the level of MRCI calculations in both [Fe(II)Cl4]2- and [Fe(III)Cl4]1- cases, very good agreement with the experimental Fe L-edge XAS spectra is obtained provided that the employed active space is extended to include ligand-based orbitals in addition to metal-based molecular orbitals. It is shown that this is necessary in order to correctly describe the important σ- and π- Fe-Cl covalent interactions. By contrast, MREOM-CC calculations yield excellent agreement relative to experiment even with small active spaces. The efficiency of the employed MR computational protocols is thoroughly discussed. Overall, we believe that this study serves as an important reference for future developments and applications of MR methods in the field of X-Ray spectroscopy.

Challenges in Multireference Perturbation Theory for the Calculations of the g-Tensor of First-Row Transition-Metal Complexes.

In this article, we have studied 34 S = 1/2 complexes of first-row transition-metal complexes in d1, d5, d7, and d9 configurations in an attempt to determine the intrinsic accuracy of the scalar-relativistic complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) methods, with respect to predicting molecular g-values. CASSCF calculations based on active spaces that contain only metal-based orbitals largely overestimate the g-values, compared to experiment and often fail to provide chemically meaningful results. Incorporation of dynamic correlation by means of the NEVPT2 method significantly improves the transition energies, with a typical error, relative to the experiment, of 2000-3000 cm-1. As a result, a lowering in the g-shift by almost an order of magnitude is obtained, relative to the CASSCF results. However, the g-shifts are still overestimated, compared to the experiment, since CASSCF leads to an overly ionic description of the metal-ligand bond and, hence, to spin-orbit coupling matrix elements that are too large. Inclusion of the double d-shell, along with appropriate bonding counterparts to the antibonding d-orbitals in the active space, led to the correct trends in the g-values for all studied complexes, with the linear regression coefficient ( R) equal to 0.93 over the entire dataset. Various technical aspects of the calculations such as the influence of relativity, importance of picture change effects, solvation effects, and comparison between second-order perturbation and effective Hamiltonian-based theories have also been systematically studied. In addition, g-tensor calculations were performed with five popular density functional theory (DFT) methods (B3LYP, M06L, M06, TPSSh, and PBE0) to compare with wave function (WF) methods. Our results suggest that WF-based methods are remarkably better than DFT methods. However, despite the fact that WF theory has come a long way in computing the properties of large, open-shell transition-metal complexes, methodological work is still necessary for truly high accuracies to be reached.

Charge Transfer Properties of Triarylamine Integrated Dimolybdenum Dyads.

Three quadruply bonded dimolybdenum complexes equipped with a triarylamine pendant, [(DAniF)3Mo2(μ-O2CC6H4N(C6H4CH3)2] (DAniF = N,N'-di(p-anisyl)formamidinate; [OO-ph-N]), [(DAniF)3Mo2(μ-OSCC6H4N(C6H4CH3)2] ([OS-ph-N]), and [(DAniF)3Mo2(μ-S2CC6H4N(C6H4CH3)2] ([SS-ph-N]), have been synthesized and characterized by single crystal X-ray diffraction. In electrochemical measurements, the redox couple for the organic amine group becomes irreversible, reflecting the substantially strong electronic interaction between the dimetal center and organic redox site. The potential difference for the two successive redox events, ca. ΔE1/2(E1/2(2)(N/N•+) - E1/2(1)(Mo2IV/V)), falls in the range of 0.5-0.8 V as estimated from the differential pulse voltammograms. For the monocation radicals [OO-ph-N]+, [OS-ph-N]+, and [SS-ph-N]+, obtained by chemical oxidation of the neutral precursor, a broad ligand (amine) to metal (Mo2) charge transfer (LMCT) absorption band is observed in the near-IR region. Interestingly, analogous to the intervalence charge transfer (IVCT) bands for mixed-valence complexes, the LMCT absorption bands, which are solvent dependent, decrease in energy and bandwidth as the electronic coupling between the two redox sites increases in an order of increasing S content in the chelating group. The electronic coupling matrix elements (Hab) are determined by optical analyses from the generalized Mulliken-Hush (GMH) theory, falling in the range of 400-800 cm-1 in CH2Cl2. These results indicate that in these radical cations the charge is localized. Time-dependent DFT calculations show that the frontier molecular orbitals for these asymmetrical donor-acceptor systems have unbalanced distribution of electron density, and the LMCT bands arise from an electronic transition from the pendant ligand-based to metal-based molecular orbitals, corresponding to donor (N)-acceptor (Mo2) charge transfer.

Photophysical Properties of Organoplatinum(II) Compounds and Derived Self-Assembled Metallacycles and Metallacages: Fluorescence and its Applications.

Over the past couple of decades, coordination-driven self-assembly has evolved as a broad multidisciplinary domain that not only covers the syntheses of aesthetically pleasing supramolecular architectures but also emerges as a method to form new optical materials, chemical sensors, theranostic agents, and compounds with light-harvesting and emissive properties. The majority of these applications depend upon investigations that reveal the photophysical nature and electronic structure of supramolecular coordination complexes (SCCs), including two-dimensional (2D) metallacycles and three-dimensional (3D) metallacages. As such, well-defined absorption and emission spectra are important for a given SCC to be used for sensing, bioimaging, and other applications with molecular fluorescence being an important component. In this Account, we summarize the photophysical properties of some bis(phosphine)organoplatinum(II) compounds and their discrete SCCs. The platinum(II) based organometallic precursors typically display spectral red-shifts and have low fluorescence quantum yields and short fluorescence lifetimes compared to their organic counterparts because the introduction of metal centers enhances both intersystem crossing (ISC) and intramolecular charge transfer (ICT) processes, which can compete with the fluorescence emissions. Likewise ligands with conjugation can also increase the ICT process; hence the corresponding organoplatinum(II) compounds undergo a further decrease in fluorescence lifetimes. The use of endohedral amine functionalized 120°-bispyridyl ligands can dramatically enhance the emission properties of the resultant organoplatinum(II) based SCCs. As such these SCCs display emissions in the visible region (ca. 400-500 nm) and are significantly red-shifted (ca. 80-100 nm) compared to the ligands. This key feature makes them suitable as supramolecular theranostic agents wherein these unique emission properties provide diagnostic spectroscopic handles and the organoplatinum(II) centers act as potential anticancer agents. Using steady state and time-resolved-spectroscopic techniques and quantum computations in concert, we have determined that the emissive properties stem from the ligand-centered transitions involving π-type molecular orbitals with modest contributions from the metal-based orbitals. The self-assembly and the photophysics of organoplatinum(II) ← 3-substituted pyridyl based SCCs are highly diverse. Subtle changes in the ligands' structures can form molecular congener systems with distinct conformational and photophysical properties. Furthermore, the heterometallic SCCs described herein possess rich photophysical properties and can be used for sensing based applications. Tetraphenylethylene (TPE) based SCCs display emissions in the aggregated state as well as in dilute solutions. This is a unique phenomenon that bridges the aggregation caused quenching (ACQ) and aggregation induced emission (AIE) effects. Moreover, a TPE based metallacage exhibits solvatoluminescence, including white light emission in THF solvent, and can act as a fluorescence-sensor for structurally similar ester compounds.

Structural, Spectral (IR and UV/Visible) and Thermodynamic Properties of Some 3d Transition Metal(II) Chloride Complexes of Glyoxime and Its Derivatives: A DFT and TD-DFT Study

Metal complexes bearing vic-dioxime ligands have been extensively used as analytical and biochemical reagents, and are well-known antimicrobial agents. Herein is reported a DFT study on the molecular structures, thermodynamic properties, chemical reactivity and spectral properties of some 3d metal(II) chloride complexes of glyoxime. The functionals B3LYP and CAM-B3LYP have each been used in conjunction with LANL2DZ for the metal(II) ions (Fe2+, Co2+, Ni2+ and Cu2+) and the Poplestyle basis set 6-31G+(d,p) for the rest of the elements, to perform theoretical calculations. The metal complexation abilities of the glyoxime ligands studied in this work have been evaluated on the basis of metal-ligand binding energies. These ligands were found to have high affinities towards Ni(II) and Fe(II) ions, and all complexation reactions were found to be thermodynamically feasible. Ligand-to-metal electron donations in the complexes studied have been revealed by natural population analysis. The fully optimized geometries of the complexes have adopted square planar structures around the central metal ions. On the basis of orbital composition analysis, the UV-Vis electronic absorption bands of these molecules have been attributed mainly to MLCT, LMCT and d-d electronic transitions involving metal-based orbitals.

AuHg(o-C6H4PPh2)2I]: A Dinuclear Heterometallic Blue Emitter

The heteronuclear AuI/HgII complex [AuHg(o-C6H4PPh2)2I] (1) was prepared by reacting of [Hg(2-C6H4PPh2)2] with [Au(tht)2]ClO4 (1:1) and NaI in excess. The heterometallic compound 1 has been structurally characterized and shows an unusual blue luminescent emission in the solid state. Theoretical calculations suggest that that the origin of the emission arises from the iodide ligand arriving at metal-based orbitals in a Ligand to Metal-Metal Charge Transfer transition.

Fitting the pieces of the puzzle: the δ bond.

The development of our understanding of the δ bond and its role in quadruple metal-metal bonding is described in terms of the conceptual advances and experimental and theoretical results achieved through a 50-year period beginning with the seminal report by Cotton and co-workers in 1964. The work behind the original discovery is described, along with the qualitative orbital description of the components of the quadruple bond. The effect of torsion about the metal-metal axis on the metal-metal bond length is described, together with the conclusion that this accords with a progressive loss of the δ component of the metal-metal bond. The important role of photoelectron spectroscopy in characterizing the loss of electrons from the metal-metal bonding orbitals is reviewed, as are the electron paramagnetic resonance results that establish that unpaired electrons, when present, populate metal-based orbitals. Other important results are described: destabilization of the metal-metal bond to produce strong reducing agents, exceptions to the expected orbital ordering, and the use of chiroptical properties to reveal additional information about the electronic structure of the metal-metal bond.

Manipulating magnetism: Ru₂⁵⁺ paddlewheels devoid of axial interactions.

Variable-temperature magnetic and structural data of two pairs of diruthenium isomers, one pair having an axial ligand and the formula Ru2(DArF)4Cl (where DArF is the anion of a diarylformamidine isomer and Ar = p-anisyl or m-anisyl) and the other one being essentially identical but devoid of axial ligands and having the formula [Ru2(DArF)4]BF4, show that the axial ligand has a significant effect on the electronic structure of the diruthenium unit. Variable temperature crystallographic and magnetic data as well as density functional theory calculations unequivocally demonstrate the occurrence of π interactions between the p orbitals of the chlorine ligand and the π* orbitals in the Ru2(5+) units. The magnetic and structural data are consistent with the existence of combined ligand σ/metal σ and ligand pπ/metal-dπ interactions. Electron paramagnetic resonance data show unambiguously that the unpaired electrons are in metal-based molecular orbitals.

Metal-Dependent Stability of Pristine and Functionalized Unconventional Dimetallofullerene M2@Ih-C80

The unstable fullerene Ih-C80 can be stabilized greatly by encapsulation of two rare-earth metal atoms in its interior. Here we present a theoretical study on the electronic structures and stabilities of dimetallofullerene M2@Ih-C80 (M = Y, La, Gd, Lu). Density functional calculations demonstrate that La2@Ih-C80 has a closed-shell configuration, whereas Y2@Ih-C80 has a triplet ground state. Unlike La2@Ih-C80 with a high stability, Y2@Ih-C80 is found to be a highly reactive radical due to the unpaired electron distributed on the fullerene cage. Such characteristic properties of La2@Ih-C80 and Y2@Ih-C80 originate from the different energy levels of the metal-based molecular orbitals. Y2@Ih-C80 can be stabilized, however, through exterior functionalization with a CF3 group. Interestingly, metallofullerenes Gd2@Ih-C80 and Lu2@Ih-C80 have similar frontier molecular orbitals and chemical stabilities to those of Y2@Ih-C80.

Synthesis, Electronic Structure, and Magnetism of [Ni(6-Mes)2]+: A Two-Coordinate Nickel(I) Complex Stabilized by Bulky N-Heterocyclic Carbenes

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior.

Electronic Ground-State and Orbital Ordering of Iron Phthalocyanine on H/Si(111) Unraveled by Spatially Resolved Tunneling Spectroscopy

The electronic properties of FePc molecules adsorbed on hydrogen-passivated Si(111) surfaces are investigated by scanning tunneling microscopy at low temperatures. Spatially resolved spectroscopy reveals a significant variation of the electronic states at different positions above the molecule. The highest occupied ligand- and metal-based orbitals of FePc are determined by pronounced peaks in the tunneling spectra and voltage-dependent changes in the microscopic images. Comparison with density functional theory calculations indicates that the electronic ground state is an 3A1g state.

Photophysical and Computational Investigations of Bis(phosphine) Organoplatinum(II) Metallacycles

A series of endohedral and exohedral amine-functionalized ligands were synthesized and used in the construction of supramolecular D(2h) rhomboids and a D(6h) hexagon. These supramolecular polygons were obtained via self-assembly of 120° dipyridyl donors with 180° or 120° diplatinum precursors when combined in 1:1 ratios. Steady-state absorption and emission spectra were collected for each ligand and metallacycle. Density functional theory (DFT) and time-dependent DFT calculations were employed to probe the nature of the observed optical transitions for the rhomboids. The emissive properties of these bis(phosphine) organoplatinum metallacycles arise from ligand-centered transitions involving π-type molecular orbitals with modest contributions from metal-based atomic orbitals. The D(2h) rhomboid self-assembled from 2,6-bis(4-pyridylethynyl)aniline and a 60° organoplatinum(II) acceptor has a low-energy excited state in the visible region and emits above 500 nm, properties which greatly differ from those of the parent 2,6-bis(4-pyridylethynyl)aniline ligand.

Very Short Metallophilic Interactions Induced by Three-Center–Two-Electron Perhalophenyl Ligands in Phosphorescent Au–Cu Complexes

The synthesis, structural characterization, and study of the photoluminescent properties of the complexes [Au3Cu(C6F5)4(N≡CCH═CHPh)3]n (1) and [Au2Cu(C6Cl5)3(N≡CCH═CHPh)2] (2) have been carried out. The crystal structures of complexes 1 and 2 display a common [Au2Cu(C6X5)2(μ-C6X5)(NCCHCHPh)2] fragment built up through a trinuclear Au2Cu unit with a perhalophenyl group acting as a three-center–two-electron bridging ligand. In the case of 1 an additional [Au(C6F5)(NCCHCHPh)] unit is also present in the asymmetric unit. Both complexes are brightly luminescent in the solid state at room temperature and at 77 K with lifetimes in the nanosecond range for 1 and in the microsecond range for 2. The presence of very short metallophilic interactions seems to be responsible for the low-energy luminescence observed. DFT and time-dependent DFT calculations agree with the experimental results and support the idea of that the origin of the emissions arises from metal-based orbitals and arrive at ligand-based orbitals as ...

Synthesis of Panchromatic Ru(II) Thienyl-Dipyrrin Complexes and Evaluation of Their Light-Harvesting Capacity

Ru(II) complexes with 5-(3-thienyl)-4,6-dipyrrin (3-TDP), containing 2,2'-bipyridine (bpy) or 4,4'-bis(methoxycarbonyl)-2,2'-bipyridine (dcmb) as coligands, have been prepared and extensively characterized. Crystal structure determination of [Ru(bpy)(2)(3-TDP)]PF(6) (1a) and [Ru(bpy)(3-TDP)(2)] (2) reveals that the 3-thienyl substituent is rotated with respect to the plane of the dipyrrinato moiety. These complexes, as well as [Ru(dcmb)(2)(3-TDP)]PF(6) (1b), act as panchromatic light absorbers in the visible range, with two strong absorption bands observable in each case. A comparison to known Ru(II) complexes and quantum-chemical calculations at the density functional theory (DFT) level indicate that the lower-energy band is due to metal-to-ligand charge transfer (MLCT) excitation, although the frontier occupied metal-based molecular orbitals (MOs) contain significant contributions from the 3-TDP moiety. The higher energy band is assigned to the π-π* transition of the 3-TDP ligand. Each complex exhibits an easily accessible one-electron oxidation. According to DFT calculations and spectroelectrochemical experiments, the first oxidation takes place at the Ru(II) center in 1a, but is shifted to the 3-TDP ligand in 1b. An analysis of MO energy diagrams suggests that complex 1b has potential to be used for light harvesting in the dye-sensitized (Grätzel) solar cell.

Electron transport through molecular wires based on a face-shared bioctahedral motif

Density functional theory in conjunction with non-equilibrium Green's functions is used to explore the electron transport properties of a series of molecules based on the face-shared bioctahedral (M2Cl9) motif. The metal-metal bond orders in the chosen molecules, [Rh2Cl9]3–, [Ru2Cl9]3– and [Mo2Cl9]3– vary from 0 (Rh) to 1 (Ru) and 3 (Mo), and the calculations indicate that there is a direct correlation between conductance and bond order. The [Mo2Cl9]3– case is particularly interesting as it is well known from crystallographic studies to be very flexible, the Mo–Mo bond length varying over a range of ∼0.35 A depending on cation. The upper limit of this range marks the point where homolytic cleavage of the δπ components of the triple bond is complete, and this has a marked impact on electron transport. The localization of the metal-based orbitals means that those on the left (source) and right (drain) sides respond very differently to applied bias, giving rise to resonance effects at particular bias voltages, and hence to negative differential resistance effects.

Noninnocence in Metal Complexes: A Dithiolene Dawn

Noninnocence in inorganic chemistry traces its roots back half a century to work that was done on metal complexes containing unsaturated dithiolate ligands. In a flurry of activity in the early 1960s by three different research groups, homoleptic bis and tris complexes of these ligands, which came to be known as dithiolenes, were synthesized, and their structural, electrochemical, spectroscopic, and magnetic properties were investigated. The complexes were notable for facile one-electron transfers and intense colors in solution, and conventional oxidation-state descriptions could not account for their electronic structures. The bis complexes were, in general, found to be square-planar, including the first examples of this geometry for paramagnetic complexes and different formal d(n) configurations. Several of the neutral and monoanionic tris complexes were found to have trigonal-prismatic coordination, the first time that this geometry had been observed in molecular metal complexes. Electronic structural calculations employing extended Hückel and other semiempirical computational methods revealed extensive ligand-metal mixing in the frontier orbitals of these systems, including the observation of structures in which filled metal-based orbitals were more stable than ligand-based orbitals of the same type, suggesting that the one-electron changes upon oxidation or reduction were occurring on the ligand rather than on the metal center. A summary of this early work is followed with a brief section on the current interpretations of these systems based on more advanced spectroscopic and computational methods. The take home message is that the early work did indeed provide a solid foundation for what was to follow in investigations of metal complexes containing redox-active ligands.

Does covalency really increase across the 5f series? A comparison of molecular orbital, natural population, spin and electron density analyses of AnCp3(An = Th–Cm; Cp = η5-C5H5)

The title compounds are studied with scalar relativistic, gradient-corrected (PBE) and hybrid (PBE0) density functional theory. The metal-Cp centroid distances shorten from ThCp(3) to NpCp(3), but lengthen again from PuCp(3) to CmCp(3). Examination of the valence molecular orbital structures reveals that the highest-lying Cp π(2,3)-based orbitals transform as 1e + 2e + 1a(1) + 1a(2). Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series such that in CmCp(3) the 5f manifold is at more negative energy than the Cp π(2,3)-based levels. Mulliken population analysis shows metal d orbital participation in the e symmetry Cp π(2,3)-based orbitals. Metal 5f character is found in the 1a(1) and 1a(2) levels, and this contribution increases significantly from ThCp(3) to AmCp(3). This is in agreement with the metal spin densities, which are enhanced above their formal value in NpCp(3), PuCp(3) and especially AmCp(3) with both PBE and PBE0. However, atoms-in-molecules analysis of the electron densities indicates that the An-Cp bonding is very ionic, increasingly so as the actinide becomes heavier. It is concluded that the large metal orbital contributions to the Cp π(2,3)-based levels, and enhanced metal spin densities toward the middle of the actinide series arise from a coincidental energy match of metal and ligand orbitals, and do not reflect genuinely increased covalency (in the sense of appreciable overlap between metal and ligand levels and a build up of electron density in the region between the actinide and carbon nuclei).

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.