Solvated cobalt(II) ions in neat 1,3-propanediamine (tn) and
n-propylamine (pa) have been characterized byelectronic absorption spectroscopy and extended X-ray absorption finestructure (EXAFS) spectroscopy. Theequilibrium between tetrahedral and octahedral geometry forcobalt(II) ion has been observed in a neat pasolution,but not in neat diamine solutions such as tn and ethylenediamine (en).The thermodynamic parameters andequilibrium constant at 298 K for the geometrical equilibrium in pawere determined to be
H![](/images/entities/deg.gif)
= -36.1 ±2.3kJ mol
-1,
S![](/images/entities/deg.gif)
= -163 ± 8 Jmol
-1 K
-1, and
K298= 6.0 × 10
-3 M
-2, where
K=[Co(pa)
62+]/{[Co(pa)
42+][pa]
2}.The equilibrium is caused by the large entropy gain in formationof the tetrahedral cobalt(II) species. Thesolventexchange of cobalt(II) ion with octahedral geometry in tn and pasolutions has been studied by the
14N NMRline-broadening method. The activation parameters and rateconstants at 298 K for the solvent exchange reactionsare as follows:
H![](/images/entities/thermod.gif)
= 49.3 ± 0.9 kJmol
-1,
S![](/images/entities/thermod.gif)
= 25 ± 3 Jmol
-1 K
-1,
V![](/images/entities/thermod.gif)
= 6.6 ± 0.3 cm
3 mol
-1 at302.1K, and
k298 = 2.9 × 10
5s
-1 for the tn exchange, and
H![](/images/entities/thermod.gif)
= 36.2 ± 1.2 kJ mol
-1,
S![](/images/entities/thermod.gif)
= 35 ± 6 J mol
-1K
-1,and
k298 = 2.0 × 10
8s
-1 for the pa exchange. By comparison of theactivation parameters with those for the enexchange of cobalt(II) ion, it has been confirmed that the kineticchelate strain effect is attributed to the largeactivation enthalpy for the bidentate chelate opening and that theenthalpic effect is smaller in the case of thesix-membered tn chelate compared with the five-membered enchelate.