Nuclear Magnetic Resonance Studies of Co(II) Complexes of Thiourea and Related Ligands

A number of tetrahedral Co(II) complexes of thiourea, N-methyl thiourea, and N, N′-dimethyl thiourea have been studied by nuclear magnetic resonance (n.m.r.). All the complexes are paramagnetic and the proton resonances show large isotropic shifts in acetone-d6 solutions. The origin of these shifts is discussed. Two sets of resonances are observed for each position and this is attributed to restricted rotation about the C—N bond. The barrier for this rotation is higher in the complexes than in the free ligands. For all complexes the exchange between free and coordinated ligand is fast at room temperature but slow below −80 °C. The complexes are dissociated to a greater or lesser extent in acetone solution and equilibrium constants and other thermodynamic data are reported for this dissociation. Entropy differences are more important than energy differences in determining the extent of dissociation. It has been demonstrated that both associative and dissociative mechanisms play a part in the ligand exchange process. The relative importance of the two mechanisms differs in the various complexes.