RESTRICTED ROTATION ABOUT A METAL ARENE BOND CAUSED BY THE STERIC EFFECTS OF PROXIMAL ETHYL GROUPS - STEREODYNAMICS OF SOME COMPLEXES OF 1,3,5-TRIETHYL-2,4,6-TRIS(TRIMETHYLSILYLMETHYL)BENZENE

1,3,5-Triethyl-2,4,6-trineopentylbenzene 5 and 1,3,5-triethyl-2,4,6-tris(trimethylsilylmethyl)benzene 6 are considered as realistic role models for the intramolecular rotational behaviour of hexaethylbenzene in its metal complexes. Empirical force-field calculations are reported for the ten diastereomeric conformers of 5 and 6 which fall into four sets depending on the number of syn interactions. Variable-temperature 75.5 MHz C-13-{H-1} NMR spectra have been recorded for dicarbonylthiocarbonyl-, dicarbonyl(eta-2-cis-cyclooctene- and dicarbonyl(triphenylphosphine)-[eta-6-1,3,5-triethyl-2,4,6-tris(trimethylsilylmethyl)benzene]molybdenum(0). Decoalescence phenomena are observed for the three complexes and line-shape-fitting studies gave the barriers for rotation about molybdenum-arene bond.