Geometric and Electronic Information from the Spectroscopy of Six-Coordinate Copper(II) Compounds

Ground State and Optical Excitations in Compounds with Tetragonal CuF64- Units: Insight into KAlCuF6 and CuFAsF6

It has been argued that AAlCuF₆ (A = K, Cs) and CuFAsF₆ are the only known crystals that exhibit compressed CuF₆^4- units due to the Jahn-Teller effect. However, no grounds for this singular behavior have yet been reported. By means of first-principles calculations on such compounds and the isomorphous compounds involving Zn²⁺ ions instead of Cu²⁺, we prove that neither the ground state nor the equilibrium geometry of CuF₆^4- complexes in KAlCuF₆ and CuFAsF₆ is the result of a Jahn-Teller effect. In contrast, it is shown that the internal electric field, E_R(r), created by the rest of the lattice ions upon the localized electrons in the complex, plays an important role in understanding this matter as well as the d-d transitions of these two compounds. The energy of an optical transition is shown to involve two contributions: the intrinsic contribution derived for the isolated CuF₆^4- unit at equilibrium and the extrinsic contribution coming from the E_R(r) field. Aside from reproduction of the experimental d-d transitions observed for KAlCuF₆, it is found that in CuFAsF₆ the b_1g(x² - y²) → a_1g(3z² - r²) transition is not the lowest one due to the stronger effects from the internal field. Interestingly, the intrinsic contribution corresponding to that transition can simply be written as β(R_eq - R_ax) where R_eq and R_ax are the equatorial and axial Cu²⁺-F^- distances and β = 2.7 eV/Å is the same for all systems involving tetragonal CuF₆^4- units and an average metal-ligand distance close to 2.03 Å. This shows the existence of a common point shared by the Jahn-Teller system KZnF₃:Cu²⁺ and other non-Jahn-Teller systems such as KAlCuF₆, CuFAsF₆, K₂ZnF₄:Cu²⁺, and Ba₂ZnF₆:Cu²⁺. Although most Jahn-Teller systems display an elongated geometry, there are however many Cu²⁺ compounds with a compressed geometry but hidden by an additional orthorhombic instability. The lack of that instability in KAlCuF₆ and CuFAsF₆ is also discussed.

Understanding the Structure and Ground State of the Prototype CuF2 Compound Not Due to the Jahn-Teller Effect

Insulating CuF₂ is considered a prototype compound displaying a Jahn-Teller effect (JTE) which gives rise to elongated CuF₆^4- units. By means of first-principles calculations together with an analysis of experimental data of both CuF₂ and Cu²⁺-doped ZnF₂, we demonstrate that such an idea is not correct. For ZnF₂:Cu²⁺, we find that CuF₆^4- units are compressed always along the Z local axis with a hole essentially in a 3 z²- r² antibonding orbital, in agreement with experimental EPR data that already underline the absence of a JTE. The structure of the monoclinic CuF₂ crystal also comes from compressed CuF₆^4- complexes, although hidden by an additional orthorhombic instability due to a negative force constant of b_2g and b_3g local modes. The associated distortion, similar to that involved in K₂CuF₄ and other layered Cu²⁺ compounds, is also shown to be developed for ZnF₂:Cu²⁺ upon increasing the copper concentration. The origin of this cooperative effect is discussed together with the differences between non-Jahn-Teller systems like ZnF₂:Cu²⁺ and CuF₂ and true Jahn-Teller systems like KZnF₃:Cu²⁺. From present results and those on layered compounds, the usual assumption of a JTE for explaining the properties of d⁹ ions in low-symmetry lattices can hardly be right.

A high-resolution XAS study of aqueous Cu(II) in liquid and frozen solutions: pyramidal, polymorphic, and non-centrosymmetric

High-resolution EXAFS (k = 18 Å(-1)) and MXAN XAS analyses show that axially elongated square pyramidal [Cu(H2O)5](2+) dominates the structure of Cu(II) in aqueous solution, rather than 6-coordinate JT-octahedral [Cu(H2O)6](2+). Freezing produced a shoulder at 8989.6 eV on the rising XAS edge and an altered EXAFS spectrum, while 1s → 3d transitions remained invariant in energy position and intensity. Core square pyramidal [Cu(H2O)5](2+) also dominates frozen solution. Solvation shells were found at ∼3.6 Å (EXAFS) or ∼3.8 Å (MXAN) in both liquid and frozen phases. However, MXAN analysis revealed that about half the time in liquid solution, [Cu(H2O)5](2+) associates with an axially non-bonding 2.9 Å water molecule. This distant water apparently organizes the solvation shell. When the 2.9 Å water molecule is absent, the second shell is undetectable to MXAN. The two structural arrangements may represent energetic minima of fluxional dissolved aqueous [Cu(H2O)5](2+). The 2.9 Å trans-axial water resolves an apparent conflict of the [Cu(H2O)5](2+) core model with a dissociational exchange mechanism. In frozen solution, [Cu(H2O)5](2+) is associated with either a 3.0 Å axial non-bonded water molecule or an axial ClO4(-) at 3.2 Å. Both structures are again of approximately equal presence. When the axial ClO4(-) is present, Cu(II) is ∼0.5 Å above the mean O4 plane. This study establishes [Cu(H2O)5](2+) as the dominant core structure for Cu(II) in water solution, and is the first to both empirically resolve multiple extended solution structures for fluxional [Cu(H2O)5](2+) and to provide direct evidence for second shell dynamics.

DFT-Based Studies on the Jahn−Teller Effect in 3d Hexacyanometalates with Orbitally Degenerate Ground States

The topology of the ground-state potential energy surface of M(CN)(6) with orbitally degenerate (2)T(2g) (M = Ti(III) (t(2g)(1)), Fe(III) and Mn(II) (both low-spin t(2g)(5))) and (3)T(1g) ground states (M = V(III) (t(2g)(2)), Mn(III) and Cr(II) (both low-spin t(2g)(4))) has been studied with linear and quadratic Jahn-Teller coupling models in the five-dimensional space of the epsilon(g) and tau(2g) octahedral vibrations (Tg[symbol: see text](epsilon(g)+tau(2g)) Jahn-Teller coupling problem (T(g) = (2)T(2g), (3)T(1g))). A procedure is proposed to give access to all vibronic coupling parameters from geometry optimization with density functional theory (DFT) and the energies of a restricted number of Slater determinants, derived from electron replacements within the t(2g)(1,5) or t(2g)(2,4) ground-state electronic configurations. The results show that coupling to the tau(2g) bending mode is dominant and leads to a stabilization of D(3d) structures (absolute minima on the ground-state potential energy surface) for all complexes considered, except for [Ti(CN)(6)](3-), where the minimum is of D(4h) symmetry. The Jahn-Teller stabilization energies for the D3d minima are found to increase in the order of increasing CN-M pi back-donation (Ti(III) < V(III) < Mn(III) < Fe(III) < Mn(II) < Cr(II)). With the angular overlap model and bonding parameters derived from angular distortions, which correspond to the stable D(3d) minima, the effect of configuration interaction and spin-orbit coupling on the ground-state potential energy surface is explored. This approach is used to correlate Jahn-Teller distortion parameters with structures from X-ray diffraction data. Jahn-Teller coupling to trigonal modes is also used to reinterpret the anisotropy of magnetic susceptibilities and g tensors of [Fe(CN)(6)](3-), and the (3)T(1g) ground-state splitting of [Mn(CN)(6)](3-), deduced from near-IR spectra. The implications of the pseudo Jahn-Teller coupling due to t(2g)-e(g) orbital mixing via the trigonal modes (tau(2g)) and the effect of the dynamic Jahn-Teller coupling on the magnetic susceptibilities and g tensors of [Fe(CN)(6)](3-) are also addressed.

Aqua Ions. 2. Structural manifestations of the Jahn-Teller effect in the beta-alums

Variable-temperature single-crystal neutron diffraction structures of the alums CsM(III)(SO(4))(2).12D(2)O, where M(III) = Ti, V, Mn, and Ga, are reported. Structural differences are highlighted by the titanium and manganese alums, which undergo cubic (Pathremacr;) to orthorhombic (Pbca) phase transitions at approximately 13 and approximately 156 K, respectively. The structural instability exhibited by these salts is interpreted as arising from cooperative Jahn-Teller interactions, and these measurements characterize the structural changes that result from the coupling between the electronic and vibrational states. Although the symmetry changes associated with the phase transformations are analogous for the Ti and Mn alums, the low-temperature geometries of the tervalent hexaaqua cations are markedly different. Whereas the MnO(6) framework is subject to a pronounced tetragonal elongation, changes in the Ti-O bond lengths are very modest; but significant changes in the O-Ti-O bond angles and in the disposition of the coordinated water molecules are identified. The large differences in the transition temperatures and in the low-temperature stereochemistries of the [Ti(OD(2))(6)](3+) and [Mn(OD(2))(6)](3+) cations are related to the sensitivity of the energies of the t(2g) (O(h)) and e(g) (O(h)) orbitals to the various asymmetric vibrations of the hexaaqua complex.

Aqua Ions. 1. The structures of the [Ru(OH(2))(6)](3+) and [V(OH(2))(6)](3+) cations in aqueous solution: an EPR and UV-Vis study

Spectroscopic data are presented for the [V(OH(2))(6)](3+) and [Ru(OH(2))(6)](3+) cations, from which inferences are drawn regarding their structures in aqueous solution. EPR and absorption spectra of solutions and glasses are supplemented by spectra of the aqua ions in various crystalline environments, and the electronic and molecular structures inter-related through elementary angular overlap model calculations. It is concluded that in aqueous solution the [Ru(OH(2))(6)](3+) cation is localized in the all-horizontal D(3)(d)geometry, whereas the structure of the [V(OH(2))(6)](3+) cation is close to T(h) symmetry. These results are consistent with the most energetically favored geometries predicted by ab initio calculations.