Solvated cobalt(II) ions in neat 1,3-propanediamine (tn) and n-propylamine (pal have been characterized by electronic absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The equilibrium between tetrahedral and octahedral geometry for cobalt(II) ion has been observed in a neat pa solution, but not in neat diamine solutions such as tn and ethylenediamine ten). The thermodynamic parameters and equilibrium constant at 298 K for the geometrical equilibrium in pa were determined to be Delta H-o = -36.1 +/- 2.3 kJ mol(-1), Delta S-o = -163 +/- 8 J mol(-1) K-1, and K-298 = 6.0 X 10(-3) M(-2), where K = [Co(pa)(6)(2+)]/{[Co(pa)(4)(2+)][pa](2)}. The equilibrium is caused by the large entropy gain in formation of the tetrahedral cobalt(II) species. The solvent exchange of cobalt(II) ion with octahedral geometry in tn and pa solutions has been studied by the N-14 NMR line-broadening method. The activation parameters and rate constants at 298 K for the solvent exchange reactions are as follows : Delta H-double dagger = 49.3 +/- 0.9 kJ mol(-1), Delta S-double dagger = 25 +/- 3 J mol(-1) K-1, Delta V-double dagger = 6.6 +/- 0.3 cm(3) mol(-1) at 302.1 K, and k(298) = 2.9 x 10(5) s(-1) for the tn exchange, and Delta H-double dagger = 36.2 +/- 1.2 kJ mol(-1), Delta S-double dagger = 35 +/- 6 J mol(-1) K-1, and k(298) = 2.0 X 10(8) s(-1) for the pa exchange. By comparison of the activation parameters with those for the en exchange of cobalt(II) ion, it has been confirmed that the kinetic chelate strain effect is attributed to the large activation enthalpy for the bidentate chelate opening and that the enthalpic effect is smaller in the case of the six-membered tn chelate compared with the five-membered en chelate.