Influence of lattice interactions on the Jahn-Teller distortion of the [Cu(H2O)6]2+ ion: dependence of the crystal structure of K2(x)Rb(2-2x)[Cu(H2O)6](SeO4)2 upon the K/Rb ratio.

The temperature dependence of the structures of a wide range of mixed-cation Tutton's salts of general formula K2(x)Rb(2-2x)[Cu(H2O)6](SeO4)2 has been determined over the temperature range 90 to 320 K. Crystals with a high proportion of potassium adopt a different structure (form B) from those with a low ratio (form A). In both forms, the [Cu(H2O)6](2+) ion has an orthorhombically distorted tetragonally elongated coordination geometry, but the long and intermediate bonds occur with a different pair of water molecules in form A compared with form B. The alkali metal is surrounded by seven close oxygen atoms in form B but eight oxygen atoms in form A, and this difference in coordination number is associated with the change in the Cu-O bond distances via the hydrogen-bonding network. For crystals with between 32 and ∼41% potassium, a relatively sharp change from form B to A occurs on cooling, and the temperature at which this occurs increases approximately linearly as the proportion of potassium falls. For the whole range of mixed crystals, the bond lengths have been determined as a function of temperature. The data have been interpreted as a thermal equilibrium of the two structural forms of the [Cu(H2O)6](2+) ion that develops gradually as the temperature increases, with this becoming more pronounced as the proportions of the two cations become more similar. The temperature dependence of the bond lengths in this thermal equilibrium has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6](2+) ion is perturbed by lattice strain interactions. The magnitude and sign of the orthorhombic component of this strain interaction depends upon the proportion of potassium to rubidium ions in the structure.

Temperature dependence of the crystal structure and g-values of trans-diaquabis(methoxyacetato)copper(II): evidence for a thermal equilibrium between complexes with tetragonally elongated and compressed geometries.

The crystal structures of trans-diaquabis(methoxyacetato)copper(II) and the isostructural nickel(II) complex have been determined over a wide temperature range. In conjunction with the reported behavior of the g-values, the structural data suggest that the copper(II) compound exhibits a thermal equilibrium between three structural forms, two having orthorhombically distorted, tetragonally elongated geometries but with the long and intermediate bonds to different atoms, and the third with a tetragonally compressed geometry. This is apparently the first reported example of a copper(II) complex undergoing an equilibrium between tetragonally elongated and compressed forms. The optical spectrum of single crystals of the copper(II) compound is used to obtain metal-ligand bonding parameters which yield the g-values of the compressed form of the complex and hence the proportions of the complex in each structural form at every temperature. When combined with estimates of the Jahn-Teller distortions of the different forms, the latter produce excellent agreement with the observed temperature dependence of the bond lengths. The behavior of an infrared combination band is consistent with such a thermal equilibrium, as is the temperature dependence of the thermal ellipsoid parameters and the XAFS. The potential surfaces of the different forms of the copper(II) complex have been calculated by a model based upon Jahn-Teller coupling. It is suggested that cooperative effects may cause the development of the population of tetragonally compressed complexes, and the crystal packing is consistent with this hypothesis, though the present model may oversimplify the diversity of structural forms present at high temperature.

Vibronic effects in Cu(II)-doped Ba2Zn(HCO2)6 x 4 H2O.

The temperature-dependent electron paramagnetic resonance (EPR) spectrum of approximately 1% Cu(II) ions doped into Ba 2Zn(HCO2)6 x 4 H2O was analyzed at the Q-band frequencies over the temperature range 100-350 K to obtain structural information about the local environment. It can be concluded that the host crystal imparts a large orthorhombic strain which mainly corresponds to a tetragonal compression imposed onto the Cu(II)O6 species. This results in a copper center which adopts an orthorhombically distorted elongated geometry with the elongated axis perpendicular to the direction of the tetragonal compression due to the host crystal. There are two possible axes of elongation, and these represent two conformers separated by approximately 320 cm(-1). The thermal population of the higher energy level averages the g values, giving the observed temperature-dependent EPR spectra. The averaging process is between vibronic levels that are localized at two different minima of a single ground-state potential energy surface. These vibronic levels correspond to vibrational levels having different electronic properties. The determination of the host lattice strain parameters from the Cu(II) EPR spectra means that the guest ion is used as a probe of the environment of the Zn(II) site. The structural data derived from the lattice strain parameters are correlated with those from the Ba 2Zn(HCO2)6 x 4 H2O crystal structure.

Geometry and Electronic Structure of CuCl(6)(4-) Polyhedra Doped into (3-Chloroanilinium)(8)[CdCl(6)]Cl(4)-An EPR and Structural Investigation.

The EPR single-crystal and powder spectra of mixed crystals of (3-chloroanilinium)(8)(Cd(1-x)Cu(x)Cl(6))Cl(4) are measured as a function of temperature and x and analyzed with respect to the geometry and bonding properties of the CuCl(6) polyhedra. These undergo strong distortions due to vibronic Jahn-Teller coupling, with the resulting tetragonal elongation being superimposed by a considerable orthorhombic symmetry component induced by a host site strain acting as a compression along the crystallographic a axis. This strain becomes apparent in the cadmium compound (x = 0), whose crystal structure is also reported [a = 8.701(2) Å, b = 13.975(2) Å, c = 14.173(2) Å, alpha = 81.62(1) degrees, beta = 72.92(1) degrees, gamma = 77.57(1) degrees, triclinic P&onemacr;, Z = 1]. A calculation of the ground state potential surface and its vibronic structure nicely reproduces the g values, Cu-Cl spacings, and ligand field data. At high copper concentrations (including x = 1), the CuCl(6) polyhedra are coupled elastically, with the long axes of neighboring polyhedra having perpendicular orientations. The elastic correlation presumably is not of the long-range antiferrodistortive type, however. Above about 55 K, the angular Jahn-Teller distortion component becomes dynamically averaged within the time scale of the EPR experiment, leading to local tetragonally compressed CuCl(6) octahedra.

Temperature Dependence of the Crystal Structure and EPR Spectrum of Bis(1,3,5-Trihydroxycyclohexane)copper(II) Tosylate. A Unified Interpretation Using a Model of Dynamic Vibronic Coupling.

The crystal structure of bis(1,3,5-trihydroxycyclohexane)copper(II) tosylate is reported at temperatures of 293, 233, 188, 163, and 93 K, as are the structures of the Zn(II) and Ni(II) analogues at room temperature for comparison. The isomorphous compounds are triclinic, space group P&onemacr;, with one formula unit in the unit cell. The unit cell parameters of the Cu compound at 293 K are a = 6.456(5) Å, b = 9.505(3) Å, c = 12.544(3) Å, alpha = 76.57(2) degrees, beta = 87.48(4) degrees, gamma = 76.65(4) degrees. The centrosymmetric ZnO(6) and NiO(6) octahedra are tetragonally compressed with a slight orthorhombic distortion. The Cu(2+) polyhedra exhibit similar geometries, but with considerably larger deviations from a regular octahedron. Two of the three independent Cu-O bond lengths and two of the g-values change significantly as a function of temperature. A model of dynamic vibronic coupling is presented which explains both the EPR and structural data. Vibronic wave functions associated with a Jahn-Teller potential energy surface modified by an orthorhombic lattice "strain" are given. The temperature dependence of the structures is calculated from the nuclear parts and that of the g-values from the electronic parts of the wave functions. The temperature dependence of the structures and g-values is also interpreted using a simpler model involving an equilibrium between two forms of the complex which differ solely in their orientation in the crystal lattice, and the results of the two approaches are compared.

Influence of lattice interactions on the Jahn-Teller distortion of the [Cu(H2O)6]2+ ion: dependence of the crystal structure of K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) upon the sulfate/selenate ratio.

The temperature dependence of the structure of the mixed-anion Tutton salt K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) has been determined for crystals with 0, 17, 25, 68, 78, and 100% sulfate over the temperature range of 85-320 K. In every case, the [Cu(H2O)6]2+ ion adopts a tetragonally elongated coordination geometry with an orthorhombic distortion. However, for the compounds with 0, 17, and 25% sulfate, the long and intermediate bonds occur on a different pair of water molecules from those with 68, 78, and 100% sulfate. A thermal equilibrium between the two forms is observed for each crystal, with this developing more readily as the proportions of the two counterions become more similar. Attempts to prepare a crystal with approximately equal amounts of sulfate and selenate were unsuccessful. The temperature dependence of the bond lengths has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6]2+ ion is perturbed by a lattice-strain interaction. The magnitude and sign of the orthorhombic component of this strain interaction depends on the proportion of sulfate to selenate. Significant deviations from Boltzmann statistics are observed for those crystals exhibiting a large temperature dependence of the average bond lengths, and this may be explained by cooperative interactions between neighboring complexes.

Temperature dependence of the crystal structure and g-values of [(HC(Ph2PO)3)2Cu](ClO4)2.2H2O: influence of dynamic Jahn-Teller coupling and lattice strain interactions.

The temperature dependence of the X-ray crystal structure and powder EPR spectrum of [(HC(Ph(2)PO)(3))(2)Cu](ClO(4))(2).2H(2)O is reported, and the structure at room temperature confirms that reported previously. Below approximately 100 K, the data imply a geometry with near elongated tetragonal symmetry for the [(HC(Ph(2)PO)(3))(2)Cu](2+) complex, but on warming the two higher Cu-O bond lengths and g-values progressively converge, and by 340 K the bond lengths correspond to a compressed tetragonal geometry. The data may be interpreted satisfactorily assuming an equilibrium among the energy levels of a Cu-O(6) polyhedron subjected to Jahn-Teller vibronic coupling and a lattice strain. However, agreement with the experiment is obtained only if the orthorhombic component of the lattice strain decreases to a negligible value as the temperature approaches 340 K.

Single-crystal neutron-diffraction study of 3.4% Zn-doped (ND4)2[Cu(D2O)6](SO4)2 at 20 K.

Doping the perdeuterated ammonium copper Tutton salt (ND4)2[Cu(D2O)6](SO4)2 [perdeuterated diammonium hexaaquacopper(II) bis(sulfate)] with Zn leads to a change in the structure from dimorph A (low density) to dimorph B (high density). This change, which accompanies a switch in the direction of the Jahn-Teller distortion, had previously been observed to occur with substitution of Zn2+ at the Cu2+ site of between 1.3 (A) and 3.4% (B). In this study, the single-crystal neutron-diffraction analysis of (ND4)2[(Cu/Zn)(D2O)6](SO4)2 at 20 K, with 3.4% Zn doping and a deuterium substitution of 85% on the H-atom sites, reveals that the structure is entirely of type B, with the Cu/Zn site at an inversion centre and with no evidence of disorder or unusual atomic displacement parameters that might occur near a phase transition boundary.

Detection and measurement of noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor using multifrequency powder EPR spectroscopy: application to cis-[(NH(3))(2)Pt(1-MeU)(2)Cu(H(2)O)(2)](SO(4)) x 4.5H(2)O (1-MeU = monoanion of 1-methyluracil).

Multifrequency continuous wave EPR spectra (4-34 GHz) on a powder of the title compound are consistent with a spin-triplet state. This arises from interaction between centrosymmetrically related pairs of copper(II) ions in the solid. The spectra at all frequencies have been simulated with a single set of spin-Hamiltonian parameters. The results show that there is noncoincidence between the principal axes of the g-matrices on each copper center and those of the zero-field splitting (D) tensor. This noncoincidence is a single rotation of 33 degrees +/- 2 degrees. The parameters from the powder spectra have been verified by a subsequent single-crystal EPR study which yielded the spin-Hamiltonian parameters g(XX) = 2.074, g(YY) = 2.093, g(ZZ) = 2.385, D(XX) = +/-0.0228 cm(-1), D(YY) = +/-0.0211 cm(-1), D(ZZ) = -/+0.0439 cm(-1) with Euler angles of alpha = 179 degrees, chi = 33.4 degrees, and gamma = 328 degrees. Analysis of the zero-field splitting tensor in terms of exchange indicates that the interaction between the pairs of copper(II) ions is almost entirely dipolar in origin. This study shows that multifrequency EPR spectroscopy on powders, coupled with spectrum simulation, can detect and measure noncoincidence between the principal axes of the g-matrix and zero-field splitting tensor, and does not necessarily require the presence of metal hyperfine interactions.

EPR spectra from "EPR-silent" species: high-frequency and high-field EPR spectroscopy of pseudotetrahedral complexes of nickel(II).

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 90-550 GHz and fields up to approximately 15 T) has been used to probe the non-Kramers, S = 1, Ni(2+) ion in a series of pseudotetrahedral complexes of general formula NiL(2)X(2), where L = PPh(3) (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), /E/ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh(3))(2)Cl(2). These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh(3))(2)Br(2), HFEPR suggested /D/ = 4.5(5) cm(-1), /E/ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh(3))(2)I(2) was unsuccessful; however, magnetic susceptibility measurements indicated /D/ = 27.9(1) cm(-1), /E/ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting ( approximately 840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.

Dynamic behavior of the Jahn-Teller distorted Cu(H(2)O)(6)(2+) ion in Cu(2+) doped Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) and the crystal structure of the host lattice.

The temperature dependence of the X- and Q-band EPR spectra of Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) containing approximately 1% Cu(2+) is reported. All three molecular g-values vary with temperature, and their behavior is interpreted using a model in which the potential surface of the Jahn-Teller distorted Cu(H(2)O)(6)(2+) ion is perturbed by an orthorhombic "strain" induced by interactions with the surrounding lattice. The strain parameters are significantly smaller than those reported previously for the Cu(H(2)O)(6)(2+) ion in similar lattices. The temperature dependence of the two higher g-values suggests that in the present compound the lattice interactions change slightly with temperature. The crystal structure of the Cs(2)[Zn(H(2)O)(6)](ZrF(6))(2) host is reported, and the geometry of the Zn(H(2)O)(6)(2+) ion is correlated with lattice strain parameters derived from the EPR spectrum of the guest Cu(2+) complex.