DYNAMIC DISORDER IN CRYSTALLINE [FE2OS(CO)(12)] AND DIRECT EVIDENCE FOR ROTATION OF THE FE2OS TRIANGLE IN THE SOLID-STATE FROM VARIABLE-TEMPERATURE X-RAY-DIFFRACTION AND C-13 MAS NMR-STUDIES

Single-crystal X-ray diffraction data have been collected for [Fs(2)Os (CO)(12)] at 120, 223, 288, 292 and 323 K. The two studies at ambient temperature (288 and 292 K) reveal a approximate to 12:1 disorder of t he metal triangle as previously reported. At the two lowest temperatur es there is no evidence of disorder, while at 323 K the ratio of the m ajor:minor component of disorder decreases significantly to approximat e to 1.4.:1. Data collected on the same crystal specimens indicate une quivocally that this disorder is dynamic in nature. Two-dimensional ex change and one-dimensional variable-temperature C-13 magic angle spinn ing (MAS) NMR spectroscopy showed that carbonyl exchange is rapid abov e 306 K in the crystalline solid. Two independent exchange processes o f similar energy are observed. The first is consistent with the crysta llographic evidence, and involves an 'in-plane' rotation of the Fe2Os triangle in steps of 60 degrees within a relatively rigid icosahedral carbonyl manifold. The second involves localised axial-equatorial exch ange in the Os(CO)(4) group. The Os L(III) and Fe K edge X-ray absorpt ion fine structure spectra are consistent with identical structures be ing present in tetrahydrofuran solution and in the solid phase.