Tautomeric fluxional process in metal complexes. Insight into cobalt(III) and pentamethylcyclopentadienylrhodium(III) complexes of 2-mercaptopyrimidine ligand

Studies have been carried out on the unusual molecular nonrigidity of Co-II I(PymS)(3) and Cp*Rh-III(PymS)(2) (PymS: 2-mercaptopyrimidine). The crystal structure of Co-III(PymS)(3) approximates to an octahedral mer isomer with 4-membered N-S chelating fashion. For Cp*Rh-III(PymS)(3), one PymS ligand bonds to the rhodium ion in an S-monodentate mode (Rh-S(2)= 2.366(1) Angstr om) while the other ligand chelates to the metal ion in an N,S-bidentate mo de (Rh-S(1)= 2.414(1); Rh-N(1) = 2.103(3) Angstrom). Even though the confor mations and configurations of both complexes are still retained in solution , an unusual nonrigidity for the protons of the PymS region is observed in the solution. The broad proton signals of Co-III(PymS)(3) exhibit a tempera ture-dependence in the range of -40 similar to 40 degrees C with a free ene rgy of activation Delta G* = 64.49 kJ/mol (40 degrees C). For Cp*Rh-III(Pym S)(2), such a fluxionality has been markedly observed in solution. This flu xional behavior can be explained in terms of "ligand tautomerism" in metal complexes containing potential tautomeric forms.