Determination of the local environment of the metals in [CrIII2CoIIO(MeCO2)6(py)3]·py (py = pyridine) by chromium and cobalt K-edge extended X-ray absorption fine structure. Geometric and charge-polarisation influences on Cr–O–Cr superexchange

Abstract

X-Ray absorption spectra at the chromium and cobalt K edges have been recorded for the metal-disordered compound [Cr^III₂Co^IIO(MeCO₂)₆(py)₃]·py (py = pyridine) and the associated extended X-ray absorption fine structure (EXAFS) analysed using curved-wave theory with the inclusion of multiple scattering for the co-ordinated pyridine and acetate groups; a corresponding study for the iron K-edge EXAFS of [Fe₃O(MeCO₂)₆(py)₃]·py was accomplished as a calibrant of the interpretive procedure. The dimensions Cr–µ₃-O 1.87, Co–µ₃-O 1.95, Cr–O (acetate) 1.98, Co–O (acetate) 2.09, Cr–N 2.26, Co–N 2.24, Cr ⋯ Cr 3.30 and Cr ⋯ Co 3.28 Å were obtained from the analysis of the EXAFS, with good agreement between the values of the Cr ⋯ Co distance obtained independently from the analyses of the chromium and cobalt EXAFS. The metal–ligand and –metal distances are in good agreement with crystallographic data on related compounds. The difference in the Cr–µ₃-O and Co–µ₃-O distances indicates that the central (µ₃) oxygen atom is slightly displaced (ca. 0.05 Å) from the centre of the metal triangle, away from the Co^II and towards the point midway between the two chromium(III) atoms. Such a structural change and the increased electron density of the central oxygen combine to produce a marked increase in the Cr^III–O–Cr^III superexchange and a reduced ligand field at the chromium(III) sites, for the Cr₂CoO system as compared with Cr^III₃O analogues.