The modulation of Jahn-Teller coupling by elastic and binding strain perturbations-a novel view on an old phenomenon and examples from solid-state chemistry.

Cations in 6-coordination with orbitally degenerate E(g) ground states, such as Cu(2+) and low-spin Co(2+), play an important role in coordination chemistry-in particular, in modern complex biochemistry. The stereochemistry and the binding properties within the basic polyhedra are the subject of pronounced modifications due to vibronic coupling in such cases, but may be also significantly influenced by what is usually called an imposed strain. The latter effect makes allowance for the general observation that the host sites into which the Jahn-Teller unstable centers are substituted are seldom of O(h) symmetry and built from six equal ligands.

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 stability and eventual lone pair distortion of the hexahalide complexes and molecules of the fifth to eighth main-group elements with one lone pair, as isolated entities or in oligomeric clusters: a vibronic coupling and DFT study.

The introduced DFT-supported vibronic coupling model together with the hardness rule indicates, for the title compounds, that the tendency toward lone pair (LP) distortions decreases with increasing coordination number (CN) and upon proceeding from fluoride to iodide as the ligands. Thus, only some hexafluoro complexes and molecules are calculated to actually undergo LP deformations; here, from the possible highest-symmetry deformations, those with C(4v) geometry possess the lowest energy, leading to the complete ablation of one ligand and, hence, explaining the nonexistence of the complexes AsF6(3-), SbF6(3-), and SF6(2-). If a lower-symmetry strain is imposed on the octahedral species, for example, induced by the simultaneous presence of terminal and bridging ligands in oligomers, the vibronic energy potential is activated.

Predictive concept for lone-pair distortions - DFT and vibronic model studies of AXn-(n-3) molecules and complexes (A = NIII to BiIII; X = F-I to I-I; n = 3-6).

The stereochemical and energetic consequences of the lone-pair effect in the title molecules and complexes have been studied by DFT calculations based on a vibronic coupling concept. The anionic complexes were examined as bare entities and, more realistically, in a polarizable charge-compensating solvent continuum. The tendency for distortions of AX3 compounds away from the high-symmetry parent geometry becomes more pronounced the larger the chemical hardness of a molecule and its constituents is; on the other hand, anionic complexes AXn-(n-3) (n = 4-6) become softer and less susceptible to distortion as compared to the corresponding AX3 molecule, the larger the coordination number and the anionic charge are.

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.

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.