A dynamic nuclear magnetic resonance study of the effects of methyl group substitution of the hexahapto-arene on the intramolecular rotations of (η ⁶ -arene)dicarbonyl(triphenylphosphine)-chromium( 0 ) and -molybdenum( 0 ) complexes

The variable-temperature 75.49 MHz 13 C-{1H} n.m.r. spectra of (η6-arene)-dicarbonyl(triphenylphosphine)metal(0) complexes (arene = C6H6 –nMen, n= 0–6, metal = Cr; arene = 1,3,5-trimethylbenzene, metal = Mo) have been observed down to 145 K. The steric requirements for intramolecular rotations for these complexes are visualised as two cones with a common apex at the centre of the metal atom. Methyl substitution of the benzene ring can lead to restricted rotation of the triphenylphosphine ligand about the metal–phosphorus bond, with ΔG200‡ca. 36.6 kJ mol–1. However, if two adjacent positions on the benzene ring are unsubstituted then rapid interconversion of the phenyl-group environments of the PPh3ligand occurs even at the lowest experimentally accessible temperature. Slowed rotation about the metal–arene bond is not observed in any of the complexes.