Variable-Temperature and -Pressure Multinuclear Magnetic Resonance Studies on Solvent Exchange of Cobalt(II), Iron(II), and Manganese(II) Ions in Ethylenediamine. Kinetic Chelate Effect and Chelate Strain Effect

Variable-temperature and -pressure NMR measurements of ethylenediamine exchange on Co²⁺, Fe²⁺, and Mn²⁺ in neat ethylenediamine (en) as a bidentate solvent have been performed by the ¹⁴N, ¹³C, and ¹H NMR line-broadening technique. The solvent exchange rate constant at 298 K (k_ex²⁹⁸/s⁻¹), activation enthalpy (ΔH^‡/kJ mor⁻¹), activation entropy (ΔS^‡/J mol⁻¹ K⁻¹), and activation volume (ΔV^‡*/cm³ mol⁻¹) for the ethylenediamine exchange reaction on each metal ion are respectively as follows: 5.4 × 10³, 56.5 ± 3.3, 16 ± 10, and 0.9 ± 0.9 for Co²⁺; 4.3 × 10⁴, 46.3 ± 1.4, −1 ± 4, and −1.2 ± 0.8 for Fe²⁺; 1.7 × 10⁶, 24.7 ± 1.6, −43 ± 5, and −0.6 ± 0.5 for Mn²⁺. The exchange rate constants are smaller than available rate constants in monodentate solvents for the corresponding metal(II) ions. The variation in values of k_ex²⁹⁸ and ΔH^‡ for the en exchange along the series from Ni²⁺ to Mn²⁺ is more drastic than that for monodentate solvent exchange. The values of ΔV^‡ in en indicate no changeover of the mechanism from a dissociative interchange (I_d) for Ni²⁺ to an associative interchange (I_a) for Mn²⁺ as observed for monodentate solvent exchange. Such characteristic features for en exchange have been explained in terms of the chelate effect and chelate strain effect of the bidentate en molecule.