Metal aquo complex

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In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. 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 aquo complexes"), but of course many complexes are known to consist of a mix of aquo and other ligands. [1] [2]

Contents

Stoichiometry and structure

Hexa-aquo complexes

Structure of an octahedral metal aquo complex. M(H2O)6 cation.png
Structure of an octahedral metal aquo complex.
Chromium(II) ion in aqueous solution. Chromium (II) ion in aqueous solution.jpg
Chromium(II) ion in aqueous solution.

Most aquo complexes are mono-nuclear, with the general formula [M(H2O)6]n+, with n = 2 or 3; they have an octahedral structure. The water molecules function as Lewis bases, donating a pair of electrons to the metal ion and forming a dative covalent bond with it. Typical examples are listed in the following table.

Complexcolourelectron config.M−O distance (Å) [3] water exchange
rate (s−1, 25 °C) [4]
M2+/3+ self-exchange
rate (M−1s−1, 25 °C)
[Ti(H2O)6]3+violet(t2g)12.0251.8×105
[V(H2O)6]2+ violet(t2g)32.12 [5] 8.7×101fast
[V(H2O)6]3+ green(t2g)21.991 [6] 5.0×102fast
[Cr(H2O)6]2+ blue(t2g)3(eg)12.06 and 2.331.2×108slow
[Cr(H2O)6]3+ violet(t2g)31.9612.4×10−6slow
[Mn(H2O)6]2+ pale pink(t2g)3(eg)22.1772.1×107
[Fe(H2O)6]2+ pale blue-green(t2g)4(eg)22.0954.4×106fast
[Fe(H2O)6]3+ pale violet(t2g)3(eg)21.9901.6×102fast [7]
[Co(H2O)6]2+ pink(t2g)5(eg)22.083.2×106
[Ni(H2O)6]2+ green(t2g)6(eg)22.053.2×104
[Cu(H2O)6]2+ blue(t2g)6(eg)31.97 and 2.305.7×109
[Zn(H2O)6]2+ colorless(t2g)6(eg)42.03-2.10fast

Tutton's salts are crystalline compounds with the generic formula (NH4)2M(SO4)2·(H2O)6 (where M = V2+, Cr2+, Mn2+, Co2+, Ni2+, or Cu2+). Alums, MM′(SO4)2(H2O)12, are also double salts. Both sets of salts contain hexa-aquo metal cations.

Tetra-aquo complexes

Silver(I) forms [Ag(H2O)4]+, a rare example of a tetrahedral aquo complex. [8] Palladium(II) and platinum(II) were once thought to form square planar aquo complexes. [9]

Octa- and nona- aquo complexes

Aquo complexes of lanthanide(III) ions are eight- and nine-coordinate, reflecting the large size of the metal centres.

Binuclear-aquo complexes

Structure of
[Co2(OH2)10] color code: red = O, white = H, blue = Co. Co2(OH2)10 dication.png
Structure of [Co2(OH2)10] color code: red = O, white = H, blue = Co.

In the binuclear ion [Co2(OH2)10]4+ each bridging water molecule donates one pair of electrons to one cobalt ion and another pair to the other cobalt ion. The Co-O (bridging) bond lengths are 213 picometers, and the Co-O (terminal) bond lengths are 10 pm shorter. [10]

The complexes [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+ contain metal-metal bonds. [8]

Hydroxo- and oxo- complexes of aquo ions

Monomeric aquo complexes of Nb, Ta, Mo, W, Mn, Tc, Re, and Os in oxidation states +4 to +7 have not been reported. [9] For example, [Ti(H2O)6]4+ is unknown: the hydrolyzed species [Ti(OH)2(H2O)n]2+ is the principal species in dilute solutions. [11] With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo-complexes.

Aquo complexes of the lanthanide cations

Lanthanide salts often or perhaps characteristically form aquo complexes. The homoleptic tricationic aquo complexes have nine water ligands. [12]

Reactions

Some reactions considered fundamental to the behavior of metal aquo ions are ligand exchange, electron-transfer, and acid-base reactions.

Water exchange

Ligand exchange involves replacement of a water ligand ("coordinated water") with water in solution ("bulk water"). Often the process is represented using labeled water 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 [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. [4] Water exchange usually follows a dissociative substitution 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 following 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)
V Cr Mn Fe Co
−0.26−0.41+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. [13]

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

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. [14] 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 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 of about 4.3:

[Cr(H2O)6]3+[Cr(H2O)5(OH)]2+ + H+

Thus, the aquo ion is a weak acid, of comparable strength to acetic acid (pKa of about 4.8). This pKa 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. [8]

In concentrated solutions, some metal hydroxo complexes undergo condensation reactions, known as olation, to form polymeric species. Many minerals 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 [Al(H2O)5OH]2+ is 20000 times faster than in [Al(H2O)6]3+.

See also

Related Research Articles

<span class="mw-page-title-main">Coordination complex</span> Molecule or ion containing ligands datively bonded to a central metallic atom

A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the coordination centre, and a surrounding array of bound molecules or ions, that are in turn known as ligands or complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.

<span class="mw-page-title-main">Inorganic chemistry</span> Field of chemistry

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<span class="mw-page-title-main">Ferric</span> The element iron in its +3 oxidation state

In chemistry, iron(III) or ferric refers to the element iron in its +3 oxidation state. Ferric chloride is an alternative name for iron(III) chloride (FeCl3). The adjective ferrous is used instead for iron(II) salts, containing the cation Fe2+. The word ferric is derived from the Latin word ferrum, meaning "iron".

<span class="mw-page-title-main">Ferrous</span> The element iron in its +2 oxidation state

In chemistry, iron(II) refers to the element iron in its +2 oxidation state. The adjective ferrous or the prefix ferro- is often used to specify such compounds, as in ferrous chloride for iron(II) chloride (FeCl2). The adjective ferric is used instead for iron(III) salts, containing the cation Fe3+. The word ferrous is derived from the Latin word ferrum, meaning "iron".

Iron(III) chloride describes the inorganic compounds with the formula FeCl3(H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated forms, which are both hygroscopic. They feature iron in its +3 oxidation state. The anhydrous derivative is a Lewis acid, while all forms are mild oxidizing agents. It is used as a water cleaner and as an etchant for metals.

<span class="mw-page-title-main">Manganese(II) chloride</span> Chemical compound

Manganese(II) chloride is the dichloride salt of manganese, MnCl2. This inorganic chemical exists in the anhydrous form, as well as the dihydrate (MnCl2·2H2O) and tetrahydrate (MnCl2·4H2O), with the tetrahydrate being the most common form. Like many Mn(II) species, these salts are pink, with the paleness of the color being characteristic of transition metal complexes with high spin d5 configurations.

<span class="mw-page-title-main">Chromium(III) chloride</span> Chemical compound

Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Rhodium(III) chloride</span> Chemical compound

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<span class="mw-page-title-main">Gadolinium(III) chloride</span> Chemical compound

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<span class="mw-page-title-main">Iron(III) nitrate</span> Chemical compound

Iron(III) nitrate, or ferric nitrate, is the name used for a series of inorganic compounds with the formula Fe(NO3)3.(H2O)n. Most common is the nonahydrate Fe(NO3)3.(H2O)9. The hydrates are all pale colored, water-soluble paramagnetic salts.

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A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula [M(H2O)n]z+. The solvation number, n, determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 and 4 of the periodic table. Lanthanide and actinide aqua ions have higher solvation numbers (often 8 to 9), with the highest known being 11 for Ac3+. The strength of the bonds between the metal ion and water molecules in the primary solvation shell increases with the electrical charge, z, on the metal ion and decreases as its ionic radius, r, increases. Aqua ions are subject to hydrolysis. The logarithm of the first hydrolysis constant is proportional to z2/r for most aqua ions.

Nickel compounds are chemical compounds containing the element nickel which is a member of the group 10 of the periodic table. Most compounds in the group have an oxidation state of +2. Nickel is classified as a transition metal with nickel(II) having much chemical behaviour in common with iron(II) and cobalt(II). Many salts of nickel(II) are isomorphous with salts of magnesium due to the ionic radii of the cations being almost the same. Nickel forms many coordination complexes. Nickel tetracarbonyl was the first pure metal carbonyl produced, and is unusual in its volatility. Metalloproteins containing nickel are found in biological systems.

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