X-ray Structures and DFT Calculations on Rhodium−Olefin Complexes: Comments on the103Rh NMR Shift−Stability Correlation
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Abstract
The low-temperature X-ray structures of bis(η2-ethene)(2,4-pentanedionato)rhodium(I) (1)and bis(η2-ethene)(1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)rhodium(I) (2) were determined. Very similar Rh−ethene coordination geometries are found in the solid state, i.e., 1, Rh−C = 2.127(5) Å, and 2, Rh−C = 2.121(3) Å, in good accord with DFT calculations, i.e., 1, Rh−C = 2.132 Å, and 2, Rh−C = 2.136 Å. The calculated 103Rh NMR chemical shifts (GIAO-B3LYP/II level) for a range of bis(η2-alkene)(2,4-pentanedionato)rhodium(I) complexes also agree well with solution NMR data. The empirical correlation between transition-metal shifts and stability constants (Öhrström, L. Comm. Inorg. Chem. 1996, 18, 305) could be confirmed for simple alkenes, since the computed relative Rh−alkene binding energies were found to correlate with δ(103Rh). In contrast, chelating or fluorinated alkenes showed large deviations from this correlation. The steric and electronic effects on the Rh−alkene bond are discussed and analyzed in terms of Bader's atoms-in-molecules theory, which revealed qualitatively different binding modes of ethene and tetrafluoroethene to rhodium: ethene forms typical π-complexes in the Dewar−Chatt−Duncanson model, whereas tetrafluoroethene complexes are on the borderline to metallacyclopropanes.
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