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)32.128.7×101fast
[V(H2O)6]3+ green(t2g)21.991 [5] 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 [6]
[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. [7] Palladium(II) and platinum(II) were once thought to form square planar aquo complexes. [8]

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. [9]

The complexes [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+ contain metal-metal bonds. [7]

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. [8] For example, [Ti(H2O)6]4+ is unknown: the hydrolyzed species [Ti(OH)2(H2O)n]2+ is the principal species in dilute solutions. [10] 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. [11]

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. [12]

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. [13] 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. [7]

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">Hydroxide</span> Chemical compound

Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO• is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.

Hydrolysis is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.

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

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

In chemistry, an acid dissociation constant is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction

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

In chemistry, iron(III) refers to the element iron in its +3 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe3+.

An oxyanion, or oxoanion, is an ion with the generic formula A
xOz−
y. Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determined by the octet rule. The corresponding oxyacid of an oxyanion is the compound H
zA
xO
y. The structures of condensed oxyanions can be rationalized in terms of AOn polyhedral units with sharing of corners or edges between polyhedra. The oxyanions adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are important in biology.

<span class="mw-page-title-main">Triflate</span> Chemical group (–OSO2CF3) or anion (charge –1)

In organic chemistry, triflate, is a functional group with the formula R−OSO2CF3 and structure R−O−S(=O)2−CF3. The triflate group is often represented by −OTf, as opposed to −Tf, which is the triflyl group, R−SO2CF3. For example, n-butyl triflate can be written as CH3CH2CH2CH2OTf.

<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">Cobalt(II) chloride</span> Chemical compound

Cobalt(II) chloride is an inorganic compound of cobalt and chlorine, with the formula CoCl
2. The compound forms several hydrates CoCl
2·nH
2O, for n = 1, 2, 6, and 9. Claims of the formation of tri- and tetrahydrates have not been confirmed. The anhydrous form is a blue crystalline solid; the dihydrate is purple and the hexahydrate is pink. Commercial samples are usually the hexahydrate, which is one of the most commonly used cobalt compounds in the lab.

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

Chromium(III) chloride (also called chromic chloride) describes any of several chemical compounds with the formula CrCl3 · xH2O, where x can be 0, 5, and 6. The anhydrous compound with the formula CrCl3 is a violet solid. The most common form of the trichloride is the dark green hexahydrate, CrCl3 · 6 H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.

<span class="mw-page-title-main">Metal ammine complex</span>

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References

  1. ↑ Mark I. Ogden and Paul D. Beer "Water & O-Donor Ligands" in Encyclopedia of Inorganic Chemistry, Wiley-VCH, 2006, Weinheim. doi : 10.1002/0470862106.ia255
  2. ↑ Lincoln, S.F.; Richens, D.T.; Sykes, A.G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN   9780080437484.
  3. ↑ For Mn(II), Fe(II), Fe(III):Sham, T. K.; Hastings, J. B.; Perlman, M. L. (1980). "Structure and Dynamic Behavior of Transition-Metal Ions in Aqueous Aolution: an EXAFS Study of Electron-Exchange Reactions". J. Am. Chem. Soc. 102 (18): 5904–5906. doi:10.1021/ja00538a033.. For Ti(III), V(III), Cr(III): Kallies, B.; Meier, R. (2001). "Electronic Structure of 3d [M(H2O)6]3+ Ions from ScIII to FeIII: A Quantum Mechanical Study Based on DFT Computations and Natural Bond Orbital Analyses". Inorg. Chem. 40 (13): 3101–3112. doi:10.1021/ic001258t. PMID   11399179.
  4. 1 2 Helm, Lothar; Merbach, André E. (2005). "Inorganic and Bioinorganic Solvent Exchange Mechanisms". Chemical Reviews. 105 (6): 1923–1959. doi:10.1021/cr030726o. PMID   15941206.
  5. ↑ Cotton, F. A.; Fair, C. K.; Lewis, G. E.; Mott, G. N.; Ross, F. K.; Schultz, A. J.; Williams, J. M. (1984). "Precise Structural Characterizations of the Hexaaquovanadium(III) and Diaquohydrogen Ions. X-ray and Neutron Diffraction Studies of [V(H2O)6][H5O2](CF3SO3)4". Journal of the American Chemical Society. 106 (18): 5319–5323. doi:10.1021/ja00330a047.
  6. ↑ Grant, M.; Jordan, R. B. (1981). "Kinetics of Solvent Water Exchange on Iron(III)". Inorganic Chemistry. 20: 55–60. doi:10.1021/ic50215a014.
  7. 1 2 3 Lincoln, S. F.; Richens, D. T.; Sykes, A. G. (2003). "Metal Aqua Ions". Comprehensive Coordination Chemistry II. Comprehensive Coordination Chemistry II. Vol. 1. pp. 515–555. doi:10.1016/B0-08-043748-6/01055-0. ISBN   9780080437484.
  8. 1 2 Persson, Ingmar (2010). "Hydrated Metal Ions in Aqueous Solution: How Regular are Their Structures?". Pure and Applied Chemistry . 82 (10): 1901–1917. doi: 10.1351/PAC-CON-09-10-22 .
  9. ↑ Han, Yin-Feng; Li, Min; Wang, Tian-Wei; Li, Yi-Zhi; Shen, Zhen; Song, You; You, Xiao-Zeng (2008). "A Novel Microporous Hydrogen-Bonding Framework Constructed with Tetrathiafulvalene Tetracarboxylate Ligand: Synthesis, Structure and Magnetic Properties". Inorganic Chemistry Communications. 11 (9): 945–947. doi:10.1016/j.inoche.2008.04.028.
  10. ↑ Baes, C.F.; Mesmer, R.E. The Hydrolysis of Cations, (1976), Wiley, New York
  11. ↑ Parker, David; Dickins, Rachel S.; Puschmann, Horst; Crossland, Clare; Howard, Judith A. K. (2002). "Being Excited by Lanthanide Coordination Complexes: Aqua Species, Chirality, Excited-State Chemistry, and Exchange Dynamics". Chemical Reviews. 102 (6): 1977–2010. doi:10.1021/cr010452+. PMID   12059260.
  12. ↑ Burgess, John (1978). Metal Ions in Solution. Chichester: Ellis Horwood. ISBN   0-85312-027-7. p. 236.
  13. ↑ Wilkins, R. G. (1991). Kinetics and Mechanism of Reactions of Transition Metal Complexes (2 ed.). Weinheim: VCH. ISBN   1-56081-125-0.
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