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A vibronic-coupling type concept is used to analyze the respective geometric and energetic situation of the hexahalide complexes and molecules of the fifth to eight main-group elements. This model accounts for the observed trends, as well as reproduces the experimental structures and energy balances, at least semiquantitatively and its predictive power exceeds that of the valence-shell electron-pair repulsion concept.

Density functional theory (DFT) calculations based on a vibronic coupling concept is employed to investigate the stereochemical and energetic consequences of the lone-pair effect in the AX(sub n)(super -(n-3)) molecules and complexes. Results suggested that the lone-pair effect is an orbital overlap phenomenon.

A model was developed for analyzing the molecular geometry and the thermal equilibrium between the vibronic wave functions of bis(1,3,5-trihydroxycyclohexane)copper(II) tosylate. The model indicated the presence of a temperature dependent equilibrium between the bond lengths of the complex. Furthermore, the regular octahedral geometry of bis(1,3,5-trihydroxycyclcohexane)copper(II) tosylate is distorted by Jahn-Teller coupling and low symmetry interactions of the crystal lattice.

Electron paramagnetic resonance experiments are performed to study the geometry, bonding properties and electronic structure of mixed crystals of the CuCl6 polyhedra. Results show that the geometry of the polyhedra undergoes strong distortions due to vibronic Jahn-Teller coupling. The data also indicate that the cooperative and electronic effects in the structure of the complex are very small.

The crystal structure of the compound (ND4)2 Cu(D2O)6 (SO4)2, 1, is characterized by time-of-flight neutron diffraction and electron paramagnetic resonance spectroscopy. At 1 bar, ~1.5 kbar, and ~3.0 kbar, the orientation of the Jahn-Teller long bonds twists Cu(D2O)6(super 2+) ion switches by 90 degrees at the 1 and 1.5 bar. The pressure dependency of the EPR spectrum indicates similar energy of the two structures at ~0.5 bar. The structure of K2 Cu(H2O)6 (SO4)2 at 15 K and 1.4 kbar remains the same at 295 K and 1 bar. At 1 bar, selenate replaces sulfate anions to effect a switch.

EPR and electronic spectra reveal that the CuCl(sub 6)(super 4-) complex in (3-Cl-an)8(CuCl6)Cl4 has a tetragonally-elongated coordination geometry with the bonding containing an orthorhombic component. X-ray crystal structure exhibits the disorder of the intermediate and long Cu-Cl bonds of the CuCl(sub 6)(super 4-) units. Metal-ligand parameters agree with bond lengths obtained from the terminal ellipsoids of the chloride ions.

The compound KAlCuF6 consists of copper(II) ion, characterized by a compressed tetragonal coordination geometry of the CuF(sub 6)(super 4-) units, which is elucidated during an analysis of the 2Az square to E square (xz, yx) transition. The measurement of the electronic spectrum of the compound over visible and infrared region forms a part of the application of the study transition. The magnetic behavior and the EPR spectrum are analyzed by the computation of the angular overlap bonding parameters which helps in determining the intensity of magnetic coupling between Cu2+ ions. The Cu2+ ions are chained to each other through the axial fluoride ligands. The experimentally obtained value of coupling constant is consistent with the theoretical value of J = -60 per cm, with the additional feature of CuFCu bridging angle shown to strongly influence this coupling.

The crystal structure and electronic spectrum of the nickel complex and their corresponding zinc(II) and cobalt(II) complexes were studied. Metal-ligand bonding in both complexes were studied using the angular overlap model. The zinc complex exhibited a distinct tetrahedral geometry which followed the prediction of the Jahn-Teller theorem. The nickel complex showed a dominant distortion component in the form of a tetrahedral elongation. An analysis of the electronic spectrum showed that the NMTP ligand was a moderately strong alpha- and pi-donor with a large naphelauxetic effect.

The optical spectroscopic properties of Mn5+ doped into ten host lattices have been studied. The influence of both chemical variation [phosphates(V), arsenates(V), and vanadates(V)] and structural variation (spodiosites, apatites, and Li3PO4-type salts) on the luminescence properties is studied in detail. By varying the host lattice, the position of the sharp luminescence band can be varied from 8300 to 8960 cm-1. Excited state splittings, luminescence lifetimes, and sideband intensities are correlated with the geometrical distortion of the MnO3-4 tetrahedron. In all the host lattices, the luminescence originates from the 1E excited state at all temperatures. It is essentially unquenched at room temperature in apatite and Li3PO4-type lattices, whereas in spodiosites up to 90% of the luminescence may be quenched at room temperature. [ABSTRACT FROM AUTHOR]

Studies reveal that CuCl(super 4-)(sub 6) ions possess normal tetragonally-compressed geometry with a crucial orthorhombic component. The disordered state of the ions in the crystal is the cause of earlier attributions of unusual tetragonally-compressed geometry, suggested from X-ray crystal structure studies. The suggested structure is validated by the optical and electron paramagnetic properties of the compound.