Metal aquo complex
Metal aquo complexes are coordination compounds containing metal ions with only water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 as a ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...

s, sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

s, and perchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...

s. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand ("homoleptic
In inorganic chemistry, a homoleptic chemical compound is a metal compound with all ligands identical. The term uses a homo prefix to indicate that something is the same for all....

 aquo complexes"), but of course many complexes are known to consist of a mix of aquo and other ligands.

Stoichiometry and structure

Most common are the octahedral complexes with the formula [M(H2O)6]2+ and [M(H2O)6]3+. Some members of this series are listed in the table below. A few aquo complexes exist with coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....

s lower than six. Palladium(II) and platinum(II), for example, form square planar aquo complexes with the stoichiometry [M(H2O)4]2+. Aquo complexes of the lanthanide trications are eight- and nine-coordinate, reflecting the large size of the metal centres.

Aquo complexes of about one third of the transition metals (Zr, Hf, Nb, Ta, Mo, W, Tc, Re, Os and Au) are either unknown or very rarely described. Aquo complexes of M4+ centres would be extraordinarily acidic. For example, [Ti(H2O)6]4+ is unknown, but [Ti(H2O)6]3+ is well characterized. This acidification is related to the stoichiometry of the Zr(IV) aquo complex [Zr4(OH)12(H2O)16]8+ (see zirconyl chloride
Zirconyl chloride
Zirconyl chloride is the inorganic compound with the formula [Zr41216]Cl812. Zirconyl chloride is a misnomer because the compound contains no oxide groups, but the term is widely used. This salt, a white solid, is the most common water-soluble derivative of zirconium.-Production and structure:The...

. Similarly, [V(H2O)6]5+ is unknown, but its conjugate base, [VO(H2O)5]2+ is highly stable. Univalent metal centres such as Cu(I) and Rh(I) rarely form complexes with water. Ag(I) form tetrahedral [Ag(H2O)4]+, a rare example of a tetrahedral aquo complex.

Some aquo complexes also contain metal-metal bond]]s. Two examples are [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+.
Complexcolourelectron config.M-O distance (Å) water exchange
rate (s−1, 25 °C)!!M2+/3+ self-exchange
rate (M−1s−1, 25 °C)
[Ti(H2O)6]3+ violet (t2g)1 2.025 1.8 x 105 n.a.
[V(H2O)6]2+ violet (t2g)3 2.12 1> fast
[V(H2O)6]3+ green (t2g)2 1.991 2> fast
[Cr(H2O)6]2+ blue (t2g)3(eg)1 2.06, 2.33 1.2 x 108 slow
[Cr(H2O)6]3+ violet (t2g)3 1.961 2.4 x 10−6 slow
[Mn(H2O)6]2+ pale pink (t2g)3(eg)2 2.177 2.1 x 107 n.a.
[Fe(H2O)6]2+ blue-green (t2g)4(eg)2 2.095 4.4 x 106 fast
[Fe(H2O)6]3+ pale yellow (t2g)3(eg)2 1.990 1.6 x 102 fast
[Co(H2O)6]2+ pink (t2g)5(eg)2 2.08 3.2 x 106 n.a.
[Ni(H2O)6]2+ green (t2g)6(eg)2 2.05 3.2 x 104 n.a.
[Cu(H2O)6]2+ blue (t2g)6(eg)3 1.97, 2.30 5.7 x 109 n.a.


Three reactions are most fundamental to the behavior of metal aquo ions: ligand exchange, electron-transfer, and acid-base reactions of the O-H bonds.

Water exchange

Ligand exchange involve replacement of a water ligand ("coordinated water") with water in solution ("bulk water"). Often the process is represented using labeled water H2O*:
[M(H2O)n]z+ + H2O* → [M(H2O)n-1(H2O*)]z+ + 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 species. Thus, the exchange rates for [Na(H2O)6]+ and [Al(H2O)6]3+ differ by a factor of 109. Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for [Al(H2O)6]3+ and [Ir(H2O)6]3+ differ by a factor of 109 also. Water exchange usually follows a dissociative substitution
Dissociative substitution
Dissociative substitution describes a pathway by which compounds interchange ligands. The term is typically applied to coordination and organometallic complexes, but resembles the Sn1 mechanism in organic chemistry. The opposite pathway is associative substitution, being analogous to Sn2 pathway...

 pathway, so the rate constants indicate first order reactions.

Electron exchange

This reaction usually applies to the interconversion of di- and trivalent metal ions, which involves the exchange of only one electron. The process is called self-exchange, meaning that the ion appears to exchange electrons with itself. The standard electrode potential for the equilibrium
[M(H2O)6]2+ + [M(H2O)6]3+ [M(H2O)6]3+ + [M(H2O)6]2+
Standard redox potential for the couple M2+, M3+ / V
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

1.51 0.77 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.

Using labels to keep track of the metals, the self-exchange process is written as:
[M(H2O)6]2+ + [M*(H2O)6]3+ → [M*(H2O)6]2+ + [M(H2O)6]3+

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. The electron transfer reaction proceeds via an outer sphere electron transfer
Outer sphere electron transfer
Outer sphere refers to an electron transfer event that occurs between chemical species that remain separate intact before, during, and after the ET event. In contrast, for inner sphere electron transfer the participating redox sites undergoing ET become connected by a chemical bridge...

. Most often large reorganizational energies are associated with changes in the population of the eg level, at least for octahedral complexes

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 pKa
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...

 of about 4.3:
[Cr(H2O)6]3+ [Cr(H2O)5(OH)]2+ + H+

Thus, the aquo ion is a weak acid
Weak acid
A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogens in a solution, donating only a partial amount of its protons to the solution...

, of comparable strength to acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

 (pKa of about 4.8). This is typical of the trivalent ions. The influence of the electronic configuration on acidity is shown by the fact that [Ru(H2O)6]3+ (pKa = 2.7) is more acidic than [Rh(H2O)6]3+ (pKa =4), 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 (t2g)5 Ru(III) centre.

In more concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation
In inorganic chemistry, olation is the process by which metal ions form polymeric oxides in aqueous solution. The phenomenon is important to understand the relationship between metal ions in aqueous solution and metal oxides, which are represented by many minerals.At low pH, many metal ions exist...

, to form polymeric species. Many mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

s, form via olation. Aquo ions of divalent metal ions are less acidic than those of trivalent cations.

The hydrolyzed species species often exhibit very different properties from the precursor hexaaquo complex. For example, water exchange in [Al(H2O)5OH]2+ is some 20000 times faster than in [Al(H2O)6]3+.
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