Static and Dynamic Distortions in Five-Coordinate Complexes of cobalt(II) and rhodium(I)

Five-coordinate complexes have been prepared, of the general formulae [CoX(L'L₃)]⁺ and [RhX(L'L₃)], where X = halogen and L'L₃ = a quadridentate tripod-like ligand containing various Group V donor atom sets. The electronic spectra and magnetism of the cobalt(II) series can be interpreted on the basis of a low-spin d⁷-ion in a trigonal bipyramidal environment distorted by a static Jahn-Teller effect. Electronic spectral data for the trigonal bipyramidal rhodium(I) series show that the low-energy ¹A → ¹E ligand field transition is split at room temperature, but that the splitting collapses on reducing the temperature. This behaviour has been attributed to the operation of a dynamic Jahn-Teller effect.     

Effet Jahn-Teller coopératif dans le systéme Mn3O4Mn2SnO4

Mn³⁺ ion (3d⁴, t³_2ge_g¹) is liable to induce, by a cooperative Jahn-Teller effect, a macroscopic distortion of the cubic spinel structure; it is so in haussmanite Mn₃O₄, a tetragonal structure. The effect of chemical composition on “tetragonal-cubic” spinel transformations in the system Mn₃O₄Mn₂SnO₄ has been studied by X-ray diffraction; the c and a′ unit-cell dimensions show an abrupt change at a critical composition beyond which the system has the cubic spinel structure. The cation distribution has been worked out from an analysis of the X-ray diffraction intensities, and a correlation between the number of Mn³⁺ ions in octahedral sites and the degree of distortion has been obtained; the values of “cation-anion” bond distances, in six coordination, show that, in this system, the oxygen octahedral distortion and the macroscopic cell distortion are in a direct relationship. The paramagnetic study always attributes the “high-spin” configuration t³_2ge_g¹ to manganese.     

Jahn–Teller coupling and the influence of strain in Tg and Eg ground and excited states – A ligand field and DFT study on halide MIIIX6 model complexes [M = TiIII–CuIII; X = F−, Cl−]

In contrast to well established experimental results of vibronic coupling effects in octahedral dⁿ complexes with E_g ground states (Cu²⁺, Ag²⁺; Cr²⁺, Mn³⁺ etc.), not much useful material is available for the Jahn–Teller (JT) effect in orbital triplet ground states. The present study is concerned with this deficiency, providing data for octahedral halide model complexes with 3dⁿ cations – in particular for Ti^III, V^III and high-spin Co^III, Ni^III with T_2g and T_1g ground states, which involve, to first-order, solely splitting of the π-antibonding t_2g MOs. Besides experimental results – structural and spectroscopic, mainly from d–d spectra – data from computations are needed for a quantitative treatment of the T_g ? (?_g + τ_2g) vibronic interaction as well as in the E_g ? ?_g coupling case (Mn^III, low-spin Ni^III); DFT was the method of choice, if only critically selected outcomes are utilised. The theoretical bases of the treatment are the dⁿ ligand field matrices in O_h, extended by the inclusion of lower-symmetry distortion parameters, and the conventional theory of vibronic coupling. Caution is needed when classifying the effects of interelectronic repulsion; DFT does not reproduce the magnitudes of the Racah parameters B, C, as deduced from the d–d spectra, properly – the presumed reasons are analysed. DFT even allows one to deduce reliable vibronic coupling constants via the analysis of orbitally degenerate excited states (Cr^III, ⁴A_2g ground state). The group-theoretical analysis of the interaction with the JT-active ?_g and τ_2g modes yields D_4h, D_3d and D_2h as the possible distortion symmetries in the case of a T_g ground state. The DFT-calculations give clear evidence, that the D_4h stationary points represent the absolute minima in the T_g ? (?_g + τ_2g) potential surface – in agreement with experiment, where available. For the first time, vibronic coupling constants, characterising JT splitting of ground and excited T_g states, can be presented for trivalent 3dⁿ cations in octahedral halide ligand fields. They turn out to be smaller by a factor of almost 3 in comparison to those, which determine the coupling in σ-antibonding e_g MOs.The tetragonal splitting of T_g states is typically only small, around 0.1 eV, and suggests that strain influences from a specific ligand arrangement and/or the presence of different ligands may modify the potential surface considerably. We have studied such effects via compounds A^IM^IIIF₄, where an elastic strain induced by the host structure, and a binding strain, due to the simultaneous existence of (largely) terminal and of bridging ligands, are active. A novel strain model, in its interplay with JT coupling, is proposed and applied – using energies from the d–d spectra, structural results and data from DFT.Chloride complexes are only known for Ti^III to Fe^III; the rather small electronegativity already of Co^III suggests a reducing ligand-to-metal (3dⁿ) electron transfer for n ≥ 6. Similarly, the low-lying ligand-to-metal charge transfer bands in the d–d spectra of the Cu^IIIF₆^3? complex and the reduced T_g ? ?_g coupling strength suggest a pronounced covalency of the Cu^III–F, and, even more distinctly, of the Cu^III–O bond, which is of interest for superconductivity. The Ni^IIIF₆^3? polyhedron possesses a low-spin configuration in the elpasolite structure. The spectroscopic evidence and the DFT data indicate, that the minimum positions of the alternative _a²A_1g(_a²E_g) and _a⁴A_2g (_a⁴T_1g) potential curves are only ≤0.02 eV apart, giving rise to interesting high-spin/low-spin phenomena. It is the strong E_g ? ?_g as compared to the T_1g ? ?_g coupling, which finally stabilises a spin-doublet ground state in D_4h.We think, that the selected class of solids is unique particularly for the study of Jahn–Teller coupling in T ground states, with model character for other systems. In our overview a procedure is sketched, which uses reliable computational results (here from DFT) for supplementing incomplete experimental data, and presents – on a semiquantitative scale – convincing statements, consistent with chemical intuition. It is also a pleading for ligand field theory, which rationalises d-d spectra in terms of chemical bonding; though the latter spectra provide frequently only rather coarse information, their assistance in the energy analysis is crucial.     

A Promising Fluorooxoborate Framework with Flexibile Capability for Diverse Cations to Enhance the Second Harmonic Generation

Fluorooxoborates as potential deep ultraviolet (DUV) nonlinear optical (NLO) materials have exhibited diverse structures and NLO properties with metal cationic changes. Herein, the general mechanisms of metal cations on band gaps and optical properties in a series of typical fluorooxoborates have been clarified. It reveals that the framework of the emblematic 18-membered ring oxyfluorides has the flexibility of being able to contain different cations spanning from alkali to d¹⁰ metals by investigating the stability of the artificial CdB₅O₇F₃ structure. Besides, introducing d¹⁰ metal cations can enhance the second harmonic generation (3.1×KH₂PO₄ (KDP), d₃₆=0.39 pm V⁻¹) and also keep a DUV spectral transparency (E_g>6.2 eV). Thus, the d¹⁰-containing fluorooxoborate exhibits a great potential to be a new DUV optical material for nonlinear light-matter interactions.     

ESR study of ordered Ti(III) clusters in frozen solutions

The ESR spectrum of exchange-coupled Ti³⁺ ions of gg_e has been observed in dissolution products of metallic titanium in HCl, evidencing the formation of chloride clusters of mutually ordered Ti³⁺ ions in d_z2-ground state in distorted tetrahedral coordination. The cooperative Jahn-Teller effect (CJTE) defines orbital ordering of the d¹ of Ti³⁺ ions in the cluster.     

EPR of dynamic Jahn-Teller distortion in CuII doped magnesium Tutton's salt

The dynamic Jahn-Teller distortion in single crystals of Cu^II doped magnesium Tutton's salt, MgK₂(SO₄)₂·6H₂O was studied by EPR spectroscopy. The directions of the g tensor axes were found to coincide with those of A at room temperature as well as at 113 K and were correlated with the MgO directions in the lattice. Two spatially distinct sites of copper were detected in the crystal. The g and A values were also calculated from the polycrystalline samples at various temperatures. A d_x²-_y² ground state has been assigned for Cu^II, as in the isomorphous zinc Tutton's salt. The Silver and Getz model was applied to understand the dynamics of Jahn-Teller (JT) activity. The system was found to undergo dynamic JT distortion at all temperatures. At low temperatures, the [Cu(H₂O)₆]²⁺ ion has three possible degenerate vibronic states corresponding to three possible directions for the long axis of the octahedron. The energy separations between the three inequivalent JT valleys were estimated as δ_1,2 = 90 and δ_1,3 = 370 cm⁻¹. The lattice strain parameter V₂ and the JT stabilization energy E_JT were also estimated as 19,140 and 1030 cm⁻¹ respectively. From the E_JT value, the JT distortion present in the system was classified as a case of “strong” coupling.     

Jahn-Teller activity in eight-coordinate Oh fx systems

The angular overlap model is used for the calculation of the Jahn-Teller coupling constants for O_h, MX₈, f^x systems in the (LSJM_J) basis. For the 5f¹ anion, [UF₈]^3?, the Γ₈ ground state is predicted to show substantial Jahn-Teller activity for coupling to the τ_2g bending mode, with E_JT comparable to ω, a result consistent with the available magnetic susceptibility data.     

A2TiF5·nH2O (A=K, Rb, or Cs; n=0 or 1): Synthesis, structure, characterization, and calculations of three new uni-dimensional titanium fluorides

Three new uni-dimensional alkali metal titanium fluoride materials, A₂TiF₅·nH₂O (A=K, Rb, or Cs; n=0 or 1) have been synthesized by hydrothermal reactions. The structures of A₂TiF₅·nH₂O have been determined by single-crystal X-ray diffraction. The Ti⁴⁺ cations have been reduced to Ti³⁺ during the synthesis reactions. All three A₂TiF₅·nH₂O materials contain novel 1-D chain structures that are composed of the slightly distorted Ti³⁺F₆ corner-sharing octahedra attributable to the Jahn-Teller distortion. The coordination environment of the alkali metal cations plays an important role to determine the degree of turning in the chain structures. Complete structural analyses, Infrared and UV-vis diffuse reflectance spectra, and thermal analyses are presented, as are electronic structure calculations.     

Emission spectra of supersonically cooled dimethyldiacetylene cations

The ā²E_u → X?²E_g emission spectra of 2,4-hexadiyne (dimethyldiacetylene) cation and its d₃- and d₆-deuterated analogues were excited by electron impact on a seeded helium supersonic free jet. Vibrational analyses are made and vibrational frequencies in both states are inferred to ±1 cm^-1. Spin-orbit splittings are observed especially in degenerate overtone bands. The observed rotational contours allow a partial rotational analysis.     

Molecular Machines: Stimulation of Cation Motion in Molecular Switches

The theoretical aspects of the mechanism of the motion of cations and ligands in molecular machines referred to as redox switches are presented. The interrelated properties of cations—the energetic, electrochemical, spectral, and magnetic properties; their propensity to form either covalent or ionic bonds; and the relative softness and hardness of cations and ligands—stimulate molecular motion. These properties determine the thermal stability and stability to destruction caused by electrochemical processes and, eventually, the maximal number of transformation cycles. The maximal efficiency of redox switches is attained when the redox reaction involves a cation with a half-filled (d ⁵, f ⁷) or complete (d ¹⁰, f ¹⁴) electronic shell. The role of the Jahn-Teller effect is considered: it is responsible for geometry distortion, which stimulates cation motion. The properties of nd and 4f cations are compared from the standpoint of their use for designing redox switches. In switches constructed on the basis of supramolecular compounds containing hard and soft moieties, softer cations (Fe²⁺, Co²⁺, Cu⁺, etc.) prefer to coordinate to soft ligands and harder cations (Fe³⁺, Co³⁺, Cu²⁺, etc.) prefer to coordinate to hard ligands. A cation moves due to the soft-hard change of its coordination sphere in the course of the redox reaction. Design of redox switches based on solid compounds with a cation in mixed oxidation state is shown to be promising. Cations can change their oxidation state with a change in temperature or pressure. The possibility of designing “magnetic switches” is considered.     

Studies on the La2−xSrxNiO4 (0 ⩽ x ⩽ 1) system

A new series of solid solutions of composition La_2?xSr_xNiO₄ (0 ? x ? 1) in which the oxidation state of nickel varies from +2 at x = 0 to +3 at x = 1 has been prepared. All the members of the system crystallize in the tetragonal K₂NiF₄ structure. The tetragonality ratio $\text{c}\text{a}$ shows a maximum at x = 0.5 which is interpreted as arising from a weak cooperative Jahn-Teller distortion due to octahedral site low-spin Ni³⁺ ions. The variation of electrical properties in the system is attributed to changes in electronic configuration of Ni³⁺.     

Der Jahn-Teller-Effekt des Mn3+-Ions in oktaedrischer Fluorkoordination. Ligandenfeldspektroskopische und magnetische Untersuchungen

Jahn-Teller Effect of Mn³⁺ Ions in Octahedral F^?-Coordination. Ligand Field Spectroscopic and Magnetic Investigations From the ligand field spectra of Mn^IIIF polyhedra in different host lattice structures [AB^IMnF₆ (elpasolite type); B^IIMnF₅; A^IMnF₄ (CsFeF₄ type)] remarkable splittings of the octahedral ⁵E_g groundstate as the consequence of a tetragonal (or lower symmetry) Jahn-Teller component are deduced. The splittings vary between 8500 and 15500 cm^?1 and can be correlated with the extent of Jahn-Teller distortion in the Mn^IIIF polyhedra. While the cooperative Jahn-Teller order is of ferrodistortive symmetry in the elpasolite-structure [exception: (NH₄)₃MnF₆], the tetragonally elongated Mn^IIIF octahedra exhibit an antiferrodistortive order in compounds A^IMnF₄, as was possible to conclude from the magnetic structures found at lower temperatures. The intensities of the sharp, spin-forbidden quintet-triplet transitions in the ligand field spectra vary strongly in dependence on the temperature and the cooperative magnetic order observed for the different classes of compounds.     

Quantum-chemical analysis of electronic structure of hemochromes and parameters of their Mössbauer spectra

Results are presented from quantum-chemical calculations of the electronic structure and parameters of the Mössbauer spectra of low-spin (S=1/2) six-coordinated ferriporphyrin complexes with axial ligands in which there are cyanide (CN) and/or pyridine (Py) ligands. On the basis of quantum-chemical calculations, a theoretical explanation has been obtained for the experimentally observed relationship of an anomalous decrease in values of quadruple splitting when the Py ligand is replaced by CN. It is tentatively concluded that²E_g is the ground term in these hemochromes. In this state, the unpaired electron occupies symmetrically the d (d_xz, d_yz) orbitals of the Fe iron.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 29–33, January–February, 1992.     

Structural Characterization of YMexMn1−xO3 (Me=Cu, Ni, Co) Perovskites

The structural properties of the YMe_xMn_1−xO₃ (Me=Cu, Ni, Co) pseudobinary oxides have been studied by X-ray diffraction and electrical measurements. The powders were prepared by solid state reaction between the corresponding oxides. The incorporation in solid solution of small divalent cations, Cu²⁺, Ni²⁺, and Co²⁺, substituting for Mn in the hexagonal YMnO₃ compound, leads to a phase transition in which a perovskite-type structure is formed. The amount of substituting cation necessary for such a transition depends on the cation nature and, to a small extent, on the ionic radius. The phase transition depends strongly on the progressive substitution of the Jahn-Teller Mn³⁺ cation and therefore of the cooperative Jahn-Teller interaction weakness. The steric influence plays a secondary role, as is shown by the very small variation of the tolerance factor, t, as a function of the cation content. The solid solutions with perovskite-type structure show semiconducting behavior. The conductivity mechanism is of a thermally activated small polaron hopping.     

Phase transitions in CuZrF6 and CrZrF6: A Mössbauer and EPR study of local and cooperative Jahn-Teller distortions

CuZrF₆ and CrZrF₆ undergo phase transitions between 100 and 450K, which are induced by crystal packing effects and changes from dynamical to static Jahn-Teller distortions of the Cu(Cr)F₆ octahedra. We analyzed in particular the transitions of the Jahn-Teller type using ⁵⁷Fe²⁺, doped into the Cu²⁺(Cr²⁺) sites, as a Mössbauer probe. The quadrupole splitting is large in the region of static distortions and essentially reflects the distortion symmetry of the host Cu(Cr)F₆ polyhedra, while it vanishes in the case of a dynamical Jahn-Teller effect. Ligand field, EPR, and magnetic data are given in addition and are discussed with respect to the structures of the host compounds and the cooperative Jahn-Teller order of the tetragonally elongated Cu(Cr)F₆ polyhedra in the low-temperature phases.     

A CNDO-MO calculation of VCl4

The electronic structure of the tetrahedral molecule VCL₄ is investigated within the CNDO-MO approximations. The metal and ligand valence orbitals, 3d, 4s, 4p; and 3s, 3p; respectively, have been systematically varied in an attempt to minimize the total energy; “optimum” V 4s(χ₄ = 1.10) and 4p(d ³ p ²) orbitals have been established, but V 3d(d ⁿ ) and Cl(-δ) valence orbitals are only seen to favor lower energy for expanded orbitals. Since determining the one-electron molecular orbital level which is occupied by the vanadium lone electron is a major aspect of this investigation, all calculations have been performed in triplicate: calculations assuming the unpaired electron occupies the 3a ₁, 2 _e and 4t ₂ molecular orbital (ground state electronic configurations² A ₁,² E, and² T ₂, respectively). The Hartree-Fock equations have been solved by Roothaan's SCF method for open shells, but off-diagonal multipliers between filled and partly filled molecular orbitals of the same symmetry have been neglected. As a qualitative estimate of the error introduced by this simplification, the pertinent overlap integrals between the eigenfunctions from calculations for the three possible configurations,² A ₁,² E, and² T ₂, are investigated as functions of the component 3d(d ⁿ ) and Cl(-δ) valence orbitals. The overlap integrals from the relevant² A ₁ and² T ₂ calculations are reasonably small, but the neglect of off-diagonal multipliers in calculations on the² E state is found to be a poor approximation. An ordering of the non-filled molecular orbitals in VCl₄ of 4t ₂ < 3a ₁ < 2e < 5t ₂ seems most consistent with the numerous calculations. This suggested ground state electronic configuration of² T ₂ introduces new aspects to the consideration of a (dynamic) Jahn-Teller effect in VCl₄. Experimental data pertinent to the electronic structure of VCl₄ has been briefly summarized, but unfortunately it is inadequate to confirm or deny the present calculations.     

Dynamische Jahn-Teller Verzerrungen und chemische Bindungsverhältnisse in orbitalentarteten Sandwichkomplexen

In order to derive information on the Jahn-Teller effect and on chemical bonding in orbitally degenerate sandwich molecules, the low-spin d⁵(²E_2g) metallocenes Mn(cp)₂ and Fe(cp) as well as the d⁷(²E_1g) metallocenes Co(cp)₂ and Ni(cp) have been diluted in a variety of diamagnetic host systems and studied by ESR. at liquid helium temperature and slightly higher temperatures. Analysis of the measured anisotropic Zeeman and hyperfine data leads to the conclusions that the Jahn-Teller distortions remain entirely dynamic in all four cases (E_JT ? hv), and that the covalent delocalization of the singly occupied degenerate metal 3d orbital over the ligand rings correlates well with the observed Jahn-Teller distortion increasing strongly along the series Fe(cp) < Mn(cp)<₂ < Co(cp)₂ < Ni(cp). This finding agrees with the expectation that the ligand components of the singly occupied e_2g(e_1g*) orbitals are mainly responsible for the dynamic e_1g(e_2g) distortions in the cyclopentadienyl rings.     

New tricyanoiron(III) building blocks for the construction of molecule-based magnets

A series of tricyanoiron(III) complexes with the general formula mer-[Fe^III(5-Xsap)(CN)₃]^2? (X = H, Me, MeO, Cl or Br, sapH₂ = N-salicylidene-o-aminophenol) have been synthesized. These complexes were characterized by IR, ESI-MS, UV/Vis, elemental analysis and magnetic measurements. The structures of (PPh₄)₂[Fe^III(sap)(CN)₃] and (PPh₄)₂[Fe^III(5-Mesap)(CN)₃] have been determined by X-ray crystallography. These low-spin d ⁵ tricyanoiron(III) complexes are potential building blocks for the construction of molecule-based magnets.     

Simple yet extraordinary: Super-polyhedra-built 3D chalcogenide framework of Cs5Ga9S16 with excellent infrared nonlinear optical performance

Non-centrosymmetric chalcogenides are attracting considerable attention as highly promising infrared nonlinear optical (IR-NLO) candidates, but it is challenging to simultaneously achieve sufficient second-harmonic-generation coefficient (d_eff > 0.5 × AgGaS₂) and large energy gap (E_g > 3.5 eV). In this work, a novel ternary chalcogenide, Cs₅Ga₉S₁₆ with an ultra-wide E_g of 4.05 eV, has been successfully obtained. This sulfide belongs to the monoclinic space group Pn (No. 7) with a novel 3D anionic [Ga₉S₁₆]^5– framework that is formed by super-polyhedral [Ga₉S₂₃] units through corner-sharing S atoms. Such a unique crystal structure displays desirable characteristics which indicate a promising IR-NLO candidate: favourable phase-matching feature, sufficient d_eff (0.7 × AgGaS₂), ultrahigh laser-induced damage threshold (31.6 × AgGaS₂) and broad transparent region (0.27−14.96 µm). In addition, systematic theoretical studies and structural analysis suggest that the desirable IR-NLO performances can be attributed to the super-polyhedral building blocks. This finding may provide useful insight into the understanding and designing other high-performance IR-NLO candidates with super-polyhedral-built structures.     

Chemical and electronic characterization of cobalt in a lanthanum perovskite. Effects of strontium substitution

Two different cobaltites, LaCoO₃ and La_0.5Sr_0.5CoO_3−δ, have been prepared and characterized by means of high energy Co K-edge and low energy O K-edge X-ray absorption spectroscopy (XAS). Even though half of the La(III) is substituted by Sr(II), little or no changes can be detected in the formal oxidation state of cobalt atoms. The presence of strontium cations induces two main effects in the chemical and electronic state of the perovskite. The charge balance with Sr(II) species is reached by the formation of oxygen vacancies throughout the network, which explains the well-known increase in the reactivity of this substituted perovskite. O K-edge XAS experiments show that the Sr(II) species induce the transitions of d electrons of cobalt cations from low to high spin configuration. We propose that this change in spin multiplicity is induced by two cooperative effects: the oxygen vacancies, creating five coordinated cobalt atoms, and the bigger size of Sr(II) cations, aligning the Co-O-Co atoms, and favoring the overlapping of π-symmetry cobalt and oxygen orbitals, reducing the splitting energy of e_g and t_2g levels.