A variable temperature H NMR study of stereochemical non-rigidity in group VI metal pentacarbonyl complexes of 1,3,5,7-Tetrathian; [M(CO)5( H2)] (M=Cr OR W)

Stereochemical non-rigidity in the complexes M(CO)₅( H₂)] (M = Cr or W) has been studied by dynamic nuclear magnetic resonance spectroscopy. In the temperature range ca. −100 to 120°C two intramolecular processes were observed, namely pyramidal atomic inversion about sulphur atoms, and a commutation of the M(CO)₅ moiety between coordination sites on different sulphur atoms. Accurate energy barriers for both processes have been obtained by detailed computer simulations of the static and the dynamic spectra. The magnitudes of ΔG≠ (298.15 K) are compared with those reported for related complexes. The exact nature of the M(CO)₅ shift cannot be unambiguously assigned from the observed spectral line-shape changes. Two possible mechanisms are proposed and discussed.