Large lanthanide-induced chemical shifts in cobalt complexes of enethials (α,β-unsaturated thioaldehydes)

Eu(fod)₃-, Yb(fod)₃- and Pr(fod)₃-induced chemical shifts of the ‘thioaldehydic’ protons in enethial ligands complexed to a cobalt cyclopentadienyl group are unusually large and in the same direction (10–30 ppm downfield per mole of shift reagent per mole of substrate). The shifts of the protons induced by Eu(fod)₃ and Pr(fod)₃ in the enethial ligands show an alternation in sign on proceeding away from the sulfur atom. In contrast to the results with the fod reagents, the ytterbium and lanthanum shift reagents Yb(thd)₃ and La(thd)₃ caused only small shifts of protons in the 2-phenylpropenethial ligand. No induced shifts with the Eu or Pr reagents were observed for a cyclopentadienyl cobalt complex of dithioglyoxal. The induced shifts in these enethial complexes may be caused by varying blends of complex formation, contact and pseudocontact shifts. Caution is advised in assigning origins to lanthanide induced shifts in such organometallic systems.