Structure of the second coordination sphere of transition metal complexes. I. Co(II) complexes with aniline and pyridine

Abstract

Geometric and thermodynamic parameters relating to the structure of labile second sphere coordination complexes formed between Co(II) chelates and aniline and pyridine molecules in carbon tetrachloride solution have been obtained. The effect of dipolar interactions, resulting from the magnetic anisotropy of the cobalt chelate, on the nuclear magnetic resonance spectra of the complexing molecules has been utilized for this purpose. Both aniline and pyridine show a preferred direction of approach perpendicular to the three fold symmetry axis of the cobalt compound used. In both cases the preferred intermolecular separation corresponds closely to that expected for a second coordination sphere ligand. The preferred orientation of the aniline molecule places the amino group adjacent to the cobalt complex and the pyridine is oriented with the γ hydrogen closest to the chelate. Enthalpy changes of −2.4 ± 0.2 kcal and −2.9 ± 0.3 kcal have been obtained for the formation of the aniline and pyridine complexes, respectively. The effect of averaging tumbling and translational motions on the dipolar shifts has been considered. The meaning of the parameters derived is discussed in the light of the very labile nature of the second coordination sphere complexes. The relevance of these results to the mechanisms of ligand substitution processes and homogeneous catalysis is emphasized.