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

Abstract

Based on the 1H-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.