DETERMINATION OF THE LOCAL ENVIRONMENT OF THE METALS IN CR(III)(2)CO(II)O(MECO(2))(6)(PY)(3)]CENTER-DOT-PY (PY=PYRIDINE) BY CHROMIUM AND COBALT K-EDGE EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE - GEOMETRIC AND CHARGE-POLARIZATION INFLUENCES ON CR-O-CR SUPEREXCHANGE

X-Ray absorption spectra at the chromium and cobalt K edges have been recorded for the metal-disordered compound [(Cr2CoII)-Co-III(MeCO(2))( 6)(py)(3)]. py (py = pyridine) and the associated extended X-ray absor ption fine structure (EXAFS) analysed using curved-wave theory with th e inclusion of multiple scattering for the co-ordinated pyridine and a cetate groups; a corresponding study for the iron K-edge EXAFS of [Fe3 O(MeCO(2))(6)(py)(3)]. py was accomplished as a calibrant of the inter pretive procedure. The dimensions Cr-mu(3)-O 1.87, Co-mu(3)-O 1.95. Cr -O (acetate) 1.98, Co-O (acetate) 2.09. Cr-N 2.26, Co-N 2.24, Cr ... C r 3.30 and Cr ... Co 3.28 Angstrom were obtained from the analysis of the EXAFS, with good agreement between the values of the Cr ... Co dis tance obtained independently from the analyses of the chromium and cob alt EXAFS. The metal-ligand and -metal distances are in good agreement with crystallographic data on related compounds. The difference in th e Cr-mu(3)-O and Co-mu(3)-O distances indicates that the central (mu(3 )) oxygen atom is slightly displaced (ca. 0.05 Angstrom) from the cent re of the metal triangle, away from the Co-II and towards the point mi dway between the two chromium(III) atoms. Such a structural change and the increased electron density of the central oxygen combine to produ ce a marked increase in the Cr-II-O-Cr-III superexchange and a reduced ligand field at the chromium(III) sites, for the Cr2CoO system as com pared with (Cr3O)-O-III analogues.