Molecular modelling of (η6-arene)-Cr(CO)3 complex systems -: The barrier to rotation about the arene-metal bond axis

Based on the H-1-NMR calculated percentage of population of mono- and disubstituted arene-tricarbonyl-chromium complexes, the preferred conformation with regard to the internal rotation about the arene-chromium axis has been theoretically studied. Analytical force field expressions have also been developed to calculate the arene-(CO)(3) potential barrier and arene-Cr bond stretching. The torsional and stretching force constants are given. The new expressions to calculate force field were derived from reactivity constants and vibrational spectroscopy. The arene-Cr(CO)(3) intermolecular forces which are responsible for the change in equilibrium ratio of stable conformations were divided into two terms, the potential barrier of the arene-(CO)(3) rotating groups, and the arene-chromium stretching energy. The stable forms determined by these forces can be changed by great attractive and/or repulsive electrostatic and steric non-bonded interactions. These non-bonded forces are also depending on the arene-metal distance, which change linearly following the resonance effect of substituents on the aromatic ring. Arene-chromium bonding forces play the most important role in the arene-tricarbonyl-chromium studied here. There are appreciable changes in the arene-chromium distances from one complex to another, and it was found to be dependent of electron density on the aromatic ring. Electron-donating or electron-withdrawing substituent characters affect directly this distance and the energy difference between stable forms. (C) 1999 Elsevier Science B.V. All rights reserved.