X-ray Structures and DFT Calculations on Rhodium−Olefin Complexes: Comments on the 103 Rh NMR Shift−Stability Correlation

The low-temperature X-ray structures of bis(η 2 -ethene)(2,4-pentanedionato)rhodium(I) ( 1 )and bis(η 2 -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 103 Rh NMR chemical shifts (GIAO-B3LYP/II level) for a range of bis(η 2 -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( 103 Rh). 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.