THE METAL-ALKENE BOND IN RH(I)(BETA-DIKETONATO)(ALKENE)(2) COMPLEXES - CORRELATION OF RH-103-NMR SHIFTS WITH STABILITY-CONSTANTS, ALKENE EXCITATION-ENERGIES, AND D-D ABSORPTION-BANDS - AN EXPERIMENTAL AND THEORETICAL-STUDY

Some new rhodium(I)-alkene complexes with (2,4-pentanedionato = acac) and with (1,1,1,5,5,5-hexafluoro-2,4-pentanedionato = hfacac) have bee n prepared and characterized by means of NMR (C-13, H-1 and Rh-103) an d UV-vis spectroscopy. It was found that Rh-103-NMR chemical shifts fo r the Rh(I)(hfacac) (alkene)(2) compounds (alkene = ethylene, cis-bute ne, and trans-butene) are linearly proportional to the wavelength of t he lowest energy UV-vis absorption band. This relation is explained by the 1/Delta E factor in the Ramsey equation. The Rh-NMR shifts for th e compounds have been found to decrease with increasing HOMO-LUMO exci tation energies for the free alkenes and with increasing equilibrium c onstants for the following reaction: Rh(acac)(ethylene)(2) + alkene re versible arrow Rh(acac)(ethylene)(alkene) + ethylene. Ab initio quantu m chemical calculations were performed for several model rhodium-alken e compounds. The calculations were executed on the CASSCF level (compl ete active space SCF) for all studied rhodium-alkene systems. In some cases, calculations correlating all valence electrons were performed, The rhodium-alkene bond was found to be very similar for ethylene and cis-butene. The rhodium-tetrafluoroethylene bond on the other hand sho wed a more pronounced back-bonding, but also good sigma donation, lead ing to a stronger bond. The results of the calculations were compared to experimental data and found to be in reasonable agreement. As expec ted, no correlation was found between the Rh-NMR shift for a compound and the charge density on the rhodium atom. On the other hand, H-1-NMR shifts and the calculated charges on the alkene protons in the rhodiu m-alkene compounds show a linear correlation. The correlation between the Rh-NMR shifts for the complexes and the stability constants is ass umed to be caused by the differences in the rhodium-alkene bond distan ces.