Mixed metal clusters containing carbyne or ketenylidene bridges: High field NMR spectroscopy as a probe for structure and mechanism

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High field NMR spectroscopy is shown to be exceedingly valuable as an aid to the determination of the structures of mixed metal clusters and also as a probe for molecular rearrangements. In particular, the incorporation of an isopropyl group into chiral organometallic clusters of the tetrahedral (M 2C 2) and square-pyramidal (M 3C 2) types allows the detection of “hidden processes” such as intramolecular racemization or ligand migrations. It is possible to slow, on the NMR time scale, local rotation of individual vertices (e.g., CpMo(CO) 2 or CO(CO) 3) in M 3CR clusters, when the capping carbynyl group is very bulky. The cation [Co 3(CO) 9CCO] + can be shown to adopt a C s rather than a C 3v geometry. Finally, the mechanism of the decarbonylation of ArCOCCo 3(CO) 9 clusters to the corresponding ArCCo 3(CO) 9 molecules proceeds via initial loss of a cobalt carbonyl ligand rather than by elimination of the ketonic carbonyl.