EXAFS evidence that the CuCl[sub 6][sup 4[minus]] ion in (3-chloroanilinium)[sub 8](CuCl[sub 6])Cl[sub 4] has an elongated rather than compressed tetragonal geometry

• Published in: Inorganic chemistry Vol. 33:7
• Main Authors: Ellis, P.J., Freeman, H.C., Hitchman, M.A., Reinen, D., Wagner, B.
• Format: Journal Article
• Language: English
• Published: United States 30-03-1994
• Subjects: 400201 - Chemical & Physicochemical Properties; ABSORPTION SPECTROSCOPY; ANIONS; CHARGED PARTICLES; CHLORIDES; CHLORINE COMPOUNDS; COPPER CHLORIDES; COPPER COMPOUNDS; COPPER HALIDES; ELECTRON SPIN RESONANCE; HALIDES; HALOGEN COMPOUNDS; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; IONS; MAGNETIC RESONANCE; MOLECULAR STRUCTURE; RESONANCE; SPECTROSCOPY; TRANSITION ELEMENT COMPOUNDS; X-RAY SPECTROSCOPY
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic00085a005

Since most Cu(II) complexes have an elongated tetragonal coordination geometry, it is interesting that a recent X-ray crystal structure analysis of (3-Cl-an)[sub 8](CuCl[sub 6])Cl[sub 4](3-Cl-an = 3-chloroanilinium) showed the centrosymmetric CuCl[sub 6][sup 4[minus]] ion to have two Cu-Cl bonds significantly shorter (2.277 [angstrom]) than the other four (2.606, 2.609 [angstrom]). It was concluded that the Jahn-Teller coupling which normally produces the tetragonally elongated geometry is here associated with a ground state in which the unpaired electron is in the d[sub z][sup 2] orbital. The electron paramagnetic resonance (EPR) spectrum of (3-Cl-an)[sub 8](CuCl[sub 6])Cl[sub 4] is, however, inconsistent with this description since it indicates a significant g shift from the free-electron value when the magnetic field is parallel to the short Cu-Cl bonds. To overcome this difficulty, it was proposed that the ground state has a significant admixture of d[sub x][sup 2]-[sub y][sup 2] due to vibronic coupling. In support of the hypothesis, it was noted that the crystallographic thermal ellipsoids of the four more distant Cl[sup [minus]] ions are elongated along the Cu-Cl bond directions. The resulting model of the complex, including the form of the low-energy mode involved in the vibronic coupling. An unequivocal choice between the alternative interpretations of the X-ray crystallographic data may be made on the basis of the extended X-ray absorption fine structure (EXAFS) of the complex. The EXAFS of the complex in the crystal depends only on the local geometry of each absorber (Cu center). The tetragonally elongated and compressed geometries should give rise to distinctly different EXAFS patterns. The authors here report EXAFS measurements on (3-Cl-an)[sub 8](CuCl[sub 6])Cl[sub 4] consistent only with a tetragonally elongated coordination geometry at the Cu atom.