In
chemistry, metal aquo complexes are
coordination compound
A coordination complex consists 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 ...
s containing metal ions with only
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
as a
ligand
In coordination chemistry, a ligand is an ion or molecule ( functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ele ...
. These complexes are the predominant
species
In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
in
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would ...
s of many metal
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...
s, such as metal
nitrate
Nitrate is a polyatomic ion with the chemical formula . Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are soluble in water. An example of an insoluble ...
s,
sulfate
The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...
s, and
perchlorate
A perchlorate is a chemical compound containing the perchlorate ion, . The majority of perchlorates are commercially produced salts. They are mainly used as oxidizers for pyrotechnic devices and to control static electricity in food packaging. ...
s. They have the general
stoichiometry
Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions.
Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
. Their behavior underpins many aspects of
environmental,
biological
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...
, 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.
Stoichiometry and structure
Hexa-aquo complexes
Most aquo complexes are mono-nuclear, with the general formula , with 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.
:
Tutton's salts are crystalline compounds with the generic formula (where , , , , , or ).
Alum
An alum () is a type of chemical compound, usually a hydrated double sulfate salt of aluminium with the general formula , where is a monovalent cation such as potassium or ammonium. By itself, "alum" often refers to potassium alum, with t ...
s, , are also double salts. Both sets of salts contain hexa-aquo metal cations.
Tetra-aquo complexes
Silver(I) forms , a rare example of a
tetrahedral
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
aquo complex.
[ Palladium(II) and platinum(II) were once thought to form square planar aquo complexes.]
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
In the binuclear ion 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.
The complexes and contain metal-metal bonds.[
]
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.[ For example, is unknown: the hydrolyzed species is the principal species in dilute solutions.][Baes, C.F.; Mesmer, R.E. ''The Hydrolysis of Cations'', (1976), Wiley, New York] 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.
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 :
:
In the absence of isotopic labeling
Isotopic labeling (or isotopic labelling) is a technique used to track the passage of an isotope (an atom with a detectable variation in neutron count) through a reaction, metabolic pathway, or cell. The reactant is 'labeled' by replacing specific ...
, 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 and differ by a factor of 109. Electron configuration is also a major factor, illustrated by the fact that the rates of water exchange for and differ by a factor of 109 also. 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:
:
:
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 Crystal field theory (CFT) describes the breaking of degeneracies of electron orbital states, usually ''d'' or ''f'' orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used ...
stabilization energy (CFSE) but manganese(III) has 3 units of CFSE.[ p. 236.]
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. 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 ''e''g 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 p''K''a of about 4.3:
:
Thus, the aquo ion is a weak acid, of comparable strength to acetic acid
Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
(p''K''a 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 () is more acidic than (), 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 (''t''2g)5 Ru(III) centre.
In 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 for understanding the relationship between metal aquo complexes and metal oxides, which are represented by ma ...
, to form polymeric species. Many mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...
s 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 is 20000 times faster than in .
See also
* Hydration number
* Ligand field theory
Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine vale ...
* Metal ammine complex
References
{{Coordination complexes
Coordination complexes
Inorganic chemistry
Water chemistry