C-13 NMR-SPECTRA OF SOME GROUP-VIB AND GROUP-VIIB TRANSITION-METAL CHALCOCARBONYL COMPLEXES

The 13C NMR spectra of the five series of chalcocarbonyl complexes, (η6-C6H6)Cr(CO)2(CX), (η6-C6H5CO2Me)Cr(CO)2(CX), (η5-C5H5)Mn(CO)2(CX), (η5-C5H4Me)Mn(CO)2(CX) and (η5-C5H5)Re(CO)2(CX) (X = O, S, Se), and some of their derivatives including several 13C-enriched species have been investigated at -30 to -50°C. The chemical shift variations observed with changes in the CX ligand suggest that the π-acceptor/σ-donor capacity of these ligands increases in the order CO < CS < CSe. Changes in the nuclear charge and in the electronic density at the central metal atom affect δ(13CS) and δ(13CO) in the same manner. The increased downfield chemical shift for δ(13CX) in the chromium and manganese series on changing X from O to S and Se is in the direction expected from considerations of Pople's paramagnetic shielding expression. © 1979.