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 N-14, C-13 and H-1 NMR line-broadening technique. The solvent exchange rate constant at 298 K (k(ex)(298)/s(-1)), activation enthalpy (Delta H-double dagger/kJ mol(-1)), activation entropy (Delta S-double dagger/J mol(-1) K-1), and activation volume (Delta V-double dagger/cm(3) mol(-1)) for the ethylenediamine exchange reaction on each metal ion are respectively as follows : 5.4 x 10(3), 56.5 +/- 3.3, 16 +/- 10, and 0.9 +/- 0.9 for Co2+; 4.3 x 10(4), 46.3 +/- 1.4, -1 +/- 4, and -1.2 +/- 0.8 for Fe2+; 1.7 x 10(6), 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(ex)(298) and Delta H-double dagger for the en exchange along the series from Ni2+ to Mn2+ is more drastic than that for monodentate solvent exchange. The values of Delta V-double dagger 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.