Water-exchange rates of aqua metal ions

First-order rate constants for exchange of one coordinated water molecule at 25 °C (¹⁷O NMR), the half-lives and Eyring Gibbs energies of activation that follow, and the ligand-field activation energy for octahedron → square pyramid.


$\ce{[Cu(H2O)6]^2+}$	d⁹	5.7 × 10⁹	1.2 × 10⁻¹⁰	17.3	−0.07	Helm & Merbach 2005
$\ce{[Cr(H2O)6]^2+}$	d⁴	1.2 × 10⁸	5.8 × 10⁻⁹	26.9	−0.07	Helm & Merbach 2005
$\ce{[Mn(H2O)6]^2+}$	d⁵	2.1 × 10⁷	3.3 × 10⁻⁸	31.2	−0.00	Helm & Merbach 2005
$\ce{[Fe(H2O)6]^2+}$	d⁶	4.4 × 10⁶	1.6 × 10⁻⁷	35.1	0.07	Helm & Merbach 2005
$\ce{[Co(H2O)6]^2+}$	d⁷	3.2 × 10⁶	2.2 × 10⁻⁷	35.9	0.13	Helm & Merbach 2005
$\ce{[Mg(H2O)6]^2+}$	d⁰	5.3 × 10⁵	1.3 × 10⁻⁶	40.3	0.00	Neely & Connick 1970
$\ce{[Ti(H2O)6]^3+}$	d¹	1.8 × 10⁵	3.9 × 10⁻⁶	43.0	0.07	Helm & Merbach 2005
$\ce{[Ni(H2O)6]^2+}$	d⁸	32000	2.2 × 10⁻⁵	47.3	0.20	Helm & Merbach 2005
$\ce{[V(H2O)6]^3+}$	d²	500	0.0014	57.6	0.13	Helm & Merbach 2005
$\ce{[Fe(H2O)6]^3+}$	d⁵	160	0.0043	60.4	−0.00	Helm & Merbach 2005
$\ce{[V(H2O)6]^2+}$	d³	87	0.008	62.0	0.20	Helm & Merbach 2005
$\ce{[Al(H2O)6]^3+}$	d⁰	1.3	0.54	72.4	0.00	Hugi-Cleary et al. 1985
$\ce{[Cr(H2O)6]^3+}$	d³	2.4 × 10⁻⁶	2.9 × 10⁵	105.1	0.20	Helm & Merbach 2005
$\ce{[Ir(H2O)6]^3+}$	d⁶ (LS)	1.1 × 10⁻¹⁰	6.3 × 10⁹	129.9	0.40	Cusanelli et al. 1996

Half-life = ln 2/k; ΔG‡ from the Eyring equation k = (k_BT/h)e^(−ΔG‡/RT). The ligand-field activation energy uses the σ-only angular overlap model (LS = low spin) and explains orders of magnitude only.

Sources

L. Helm & A. E. Merbach, “Inorganic and bioinorganic solvent exchange mechanisms”, Chem. Rev. 105, 1923–1959 (2005): first transition series.
D. Hugi-Cleary, L. Helm & A. E. Merbach, Helv. Chim. Acta 68, 545–554 (1985): Al³⁺.
J. Neely & R. Connick, J. Am. Chem. Soc. 92, 3476–3478 (1970): Mg²⁺.
A. Cusanelli, U. Frey, D. T. Richens & A. E. Merbach, J. Am. Chem. Soc. 118, 5265–5271 (1996): Ir³⁺.

Used in these lessons

Ligand substitution: labile and inert