Ligandbewegungen in űbergangsmetallkomplexen XI 1H-NMR-spektroskopische untersuchungen an acetylacetonat-bis(olefin)-rhodium(I)-komplexen

The temperature-dependent ¹H NMR spectra of acetylacetonatebis(ethylene)-rhodium, (acac)Rh(C₂H₄)₂ (I), of some related complexes containing methoxy-substituted ethylenes have been measured in toluene-d₈ solution. Both monosubstituted (acac)Rh(C₂H₄)(olefin), olefin = tetramethoxyethylene (II), $\text{cis}$- and $\text{trans}$-dimethoxyethylene (III and IV)] and disubstituted (acac)Rh(olefin)₂, olefin = $\text{cis}$- and $\text{trans}$-dimethoxyethylene (V and VI), methyl vinyl ether (VII)] derivatives of I have been investigated with respect to hindered intramolecular movements of the ligands. The barriers of olefin rotation increase with an increasing number of methoxy substituents. When the olefin rotation is frozen out; the methoxy substituents of the olefins tend to be turned away from the acetylacetonate ligand unless steric interaction occurs between the two π-coordinated olefins. A hindered movement of the acetylacetonate ligand has been observed in II and V. For this movement which is independent of the olefin rotation, a degenerate rearrangement is proposed of the tetragonal-planar complexes via a tetrahedral transition state.