Observations on the structure of the ion pair formed between [Co(en)₃]³⁺ and [Co(edta)]^-

Ion pairing phenomena in solutions involving the metal-ion complexes [Co(en)₃]³⁺ (en = 1,2-diaminoethane) and [Co(edta)]^- (edta^4- = 1,2-diaminoethane-N,N,N′,N′-tetraacetate(4-)) are investigated by using nuclear magnetic resonance techniques. Specific orientation information for the [Co(edta)]^- complex is obtained by high-resolution ¹H NMR T₁ experiments with [Cr(en)₃]³⁺, an isostructural analogue for the cobalt(III) complex, as a paramagnetic probe. The correlation time, τ_c, for the [Co(edta)]^-/[Co(en)₃]³⁺ ion pair is determined to be 8.5 × 10^-11 s, significantly longer than the values for the free ions. Various observed ¹H([Co(edta)]^-) to chromium(III) metal center distances are reported, and the metal-metal distance in the ion pair is calculated to be 3.5 A. Although the absolute distances are unrealistically short, the orientation for approach of [Cr(en)₃]³⁺ to the [Co(edta)]^- complex is obtained. In agreement with earlier results from stereoselectivity studies, the interaction of [Co(edta)]^- in the ion pair is well-defined, presenting a pseudo-C₃ carboxylate face to its cationic partner.