Metal aquo complex

 ( Aqua Complexes ) 

$\backslash ce\{M(H2O)\_\backslash mathit\{n\}^\backslash mathit\{z\}+\}\; +\; \backslash ce\{H2O^\backslash star\; \}\; \backslash longrightarrow\; \backslash ce\{M(H2O)\_\backslash mathit\{n-1\}(H2O^\backslash star^\backslash mathit\{z\}+\}\; +\; \backslash ce\{H2O\}$

In the absence of isotopic labeling, the reaction is degenerate, meaning that the free energy change is zero.

Rates vary over many orders of magnitude. The main factor affecting rates is charge: highly charged metal aquo cations exchange their water more slowly than singly charged cations. Thus, the exchange rates for and differ by a factor of 10⁹. Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for and differ by a factor of 10⁹ also. Water exchange usually follows a dissociative substitution pathway, so the rate constants indicate first order reactions.

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+ Standard redox potential for the couple M2+, M3+ (V) ! vanadium!!chromium!!manganese!!iron!!cobalt

+1.82

shows the increasing stability of the lower oxidation state as atomic number increases. The very large value for the manganese couple is a consequence of the fact that octahedral manganese(II) has zero crystal field stabilization energy (CFSE) but manganese(III) has 3 units of CFSE.

John Burgess (1978). 9780853120278, Ellis Horwood. ISBN 9780853120278

p. 236.

Using labels to keep track of the metals, the self-exchange process is written as:

$\backslash ce\{M(H2O)6^2+\}\; +\; \backslash ce\{M^\backslash star^3+\}\; \backslash longrightarrow\; \backslash ce\{M^\backslash star^3+\}\; +\; \backslash ce\{M(H2O)6^2+\}$

The rates of electron exchange vary widely, the variations being attributable to differing reorganization energies: when the 2+ and 3+ ions differ widely in structure, the rates tend to be slow.

(1991). 9781560811251, VCH. ISBN 9781560811251

The electron transfer reaction proceeds via an outer sphere electron transfer. Most often large reorganizational energies are associated with changes in the population of the e_g level, at least for octahedral complexes.

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Acid–base reactions

Solutions of metal aquo complexes are acidic owing to the ionization of protons from the water ligands. In dilute solution chromium(III) aquo complex has a p K_a of about 4.3, affording a metal hydroxo complex:

Thus, the aquo ion is a weak acid, of comparable strength to acetic acid (p K_a of about 4.8). This pK_a is typical of the trivalent ions. The influence of the electronic configuration on acidity is shown by the fact that () is more acidic than (), despite the fact that Rh(III) is expected to be more electronegative. This effect is related to the stabilization of the pi-donor hydroxide ligand by the ( t_2g)⁵ Ru(III) centre.

(2025). 9780080437484 ISBN 9780080437484

In concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation, to form polymeric species. Many are assumed to form via olation. Aquo ions of divalent metal ions are less acidic than those of trivalent cations.

The hydrolyzed species often exhibit very different properties from the precursor hexaaquo complex. For example, water exchange in is 20000 times faster than in .

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See also

• Hydration number
• Ligand field theory
• Metal ammine complex
• Metal ions in aqueous solution

Categories: Aqua Complexes, Inorganic Chemistry, Water Chemistry