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 been prepared and characterized by means of NMR (C-13, H-1 and Rh-103) and UV-vis spectroscopy. It was found that Rh-103-NMR chemical shifts for the Rh(I)(hfacac) (alkene)(2) compounds (alkene = ethylene, cis-butene, and trans-butene) are linearly proportional to the wavelength of the 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 the compounds have been found to decrease with increasing HOMO-LUMO excitation energies for the free alkenes and with increasing equilibrium constants for the following reaction : Rh(acac)(ethylene)(2) + alkene reversible arrow Rh(acac)(ethylene)(alkene) + ethylene. Ab initio quantum chemical calculations were performed for several model rhodium-alkene compounds. The calculations were executed on the CASSCF level (complete 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 showed a more pronounced back-bonding, but also good sigma donation, leading to a stronger bond. The results of the calculations were compared to experimental data and found to be in reasonable agreement. As expected, 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 rhodium-alkene compounds show a linear correlation. The correlation between the Rh-NMR shifts for the complexes and the stability constants is assumed to be caused by the differences in the rhodium-alkene bond distances.