Chemistry - Do all metal salts in aqueous solution contain metal aquo complexes?

Solution 1:

[OP] But do all metal salts in aqueous solution contain such complex ions?

This is a good question and in many cases, the exact structure and dynamics of aqueous metal ions is still being studied. In the simplest terms, if something is in aqueous solution, it will be surrounded by water. To start getting some more details, take a look at the Wikipedia article on metal aquo complexes. It says that metals surrounded by a first shell of six waters (with an octahedral geometry) is very common. They also point out that some of these complexes are long-lived while others are not:

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 species. Thus, the exchange rates for $\ce{[Na(H2O)6]+ and [Al(H2O)6]^3+}$ differ by a factor of $\pu{e9}$. Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for $\ce{[Al(H2O)6]3+ and [Ir(H2O)6]^3+}$ differ by a factor of $\pu{e9}$ also.[3] Water exchange usually follows a dissociative substitution pathway, so the rate constants indicate first order reactions.

Type of metal

[OP] Is the metal required to be a transition metal?

For sodium ions in aqueous solutions, just calling them $\ce{Na+(aq)}$ is probably sufficient because the waters change places so rapidly. For a nickel complex with waters in the first shell not exchanging that quickly, it makes more sense to talk of a aquo complex and to study its geometry.

Other coordination numbers and geometries

If you dig deeper, you will encounter other coordination numbers and geometries. For example, according to this report, copper(II) likes to surround itself with five ligands in a square pyramidal fashion. Here is an excerpt of the abstract:

Cu K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near-edge structure (MXAN) analyses were combined to evaluate the structure of the copper(II) imidazole complex ion in liquid aqueous solution. [...] This core square-pyramidal motif has persisted through [Cu(H2O)5]2+, [Cu(NH3)4(NH3,H2O)]2+,(1, 2) and now [Cu(Im)4Lax)]2+ and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.

I think for geometries that are not the optimal space-filling ones, you need transition metals, but I have not done a careful survey of the available literature.

Solution 2:

In certain cases the metal cation is complexed (at least predominantly) with a soft-base solute that is also in solution.

As a graduate student, I had some copper (II) salts and thiourea in the lab, and I mixed them together in water solution to see how thiourea might chemically alter a copper electroplating bath (where the thiourea is used an an additive).

Thiourea + copper(II) sulfate --> solution turned colorless, copper (II) was reduced to copper(I)

Thiourea + copper(II) chloride -> white precipitate, copper(I) chloride sparingly soluble when only an aquo complex could be formed; but more thiourea gave a clear solution indicating copper(I) redissolved as a thiourea complex.