X-ray structures and DFT calculations on rhodium-olefin complexes: Comments on the Rh-103 NMR shift-stability correlation

The low-temperature X-ray structures of bis(eta (2)-ethene)(2,4-pentanedion ato)rhodium(I) (1)- and bis(eta (2)-ethene)(1,1,1,5,5,5-hexafluoro-2,4-pent anedionato)rhodium(I) (2) were determined. Very similar Rh-ethene coordinat ion geometries are found in the solid state, i.e., 1, Rh-C = 2.127(5) Angst rom, and 2, Rh-C = 2.121(3) Angstrom, in good accord with DFT calculations, i.e., 1, RB-C = 2.132 Angstrom and 2, Rh-C = 2.136 Angstrom. The calculate d Rh-103 NMR chemical shifts (GIAO-B3LYP/II level) for a range of bis(eta ( 2)-alkene)(2,4-pentanedionato)rhodium(I) complexes also agree well with sol ution NMR data. The empirical correlation between transition-metal shifts a nd stability constants (Ohrstrom, L. Comm. Inorg. Chem. 1996, 18, 305) coul d be confirmed for simple alkenes, since the computed relative Rh-alkene bi nding energies were found to correlate with delta(Rh-103). I, contrast, che lating 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 qua litatively different binding modes of ethene and tetrafluoroethene to rhodi um: ethene forms typical pi -complexes in the Dewar-Chatt-Duncanson model, whereas tetrafluoroethene complexes are on the borderline to metallacyclopr opanes.