I know that nickel nitrate in aqueous solution contains the metal aquo complex ion $\ce{[Ni(H2O)6]^2+}$. But do all metal salts in aqueous solution contain such complex ions? If not, on what factors does the formation of metal aquo complex ions depend? Is the metal required to be a transition metal?

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    $\begingroup$ Hint: If you dissolve an ionic substance in water, the total solution enthalpy has several components. One of them is the ... ? $\endgroup$
    – Karl
    Apr 26, 2020 at 10:38
  • $\begingroup$ @Karl sorry , I'm not a chemist at all. I'm asking because I read about it and am interested in it, and trying to understand if complexes always appear or if it's a special thing $\endgroup$
    – Katharina
    Apr 26, 2020 at 14:39
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    $\begingroup$ Your question is awfully chemically specific, for a non-chemist. ;-) What I meant is the hydration enthalpy of each of the ions involved. $\endgroup$
    – Karl
    Apr 26, 2020 at 15:26
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    $\begingroup$ Umm, you're underestimating things there. Even in nickel nitrate there's also gonna be some coordination by nitrate, and with other "salts" it's usually more pronounced. In general there's no such thing as bare monoatomic cation (or even polyatomic, to some extent) outside of stuff like plasma. $\endgroup$
    – Mithoron
    Apr 26, 2020 at 17:48
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    $\begingroup$ OK, this is really hard to explain in a few sentences. Every ion in water is surrounded by shells of water molecules. Those that are closest are bound more strongly, those in a "second shell" less so, etc, The further out, the more they are like pure water, and exchange freely. Where to draw the line is somewhat arbitrary. Typically some transition metals can include electron pairs from e.g. water into their own electronic structure, these classic "complex bonds" are usually stronger than what an alkali metal has in its hydrate shell. Of course your Ni complex ion has another hydrate shell. $\endgroup$
    – Karl
    Apr 26, 2020 at 20:10

2 Answers 2


[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.

  • $\begingroup$ I would note that in some compounds dissolved in aqueous solution, such as $\ce{K3[Fe(CN)6]}$, some metal ions will stay coordinated by ligands other than water. Of course, these are probably not the salts the question was talking about. However, we should not let the notion arise that aqueous solution equals aqua ligands. $\endgroup$
    – TAR86
    May 11, 2020 at 19:12

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.


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