Aquo complex

Aquo Complex

An aquo complex is a type of coordination compound in which water molecules are directly bonded to a central metal ion. These complexes are a subset of coordination compounds, which are molecules that consist of a central atom or ion, typically a metal, surrounded by molecules or anions called ligands. In aquo complexes, the ligands are water molecules.

Structure and Bonding

In an aquo complex, the water molecules act as ligands and coordinate to the metal ion through the oxygen atom. The oxygen atom donates a pair of electrons to the metal ion, forming a coordinate covalent bond. The number of water molecules that can coordinate to a metal ion depends on the size, charge, and electronic configuration of the metal ion.

For example, in the hexaaquacopper(II) ion, \([\text{Cu(H}_2\text{O})_6]^{2+}\), six water molecules are coordinated to a central copper ion. The geometry of this complex is typically octahedral, which is common for six-coordinate complexes.

Properties

Aquo complexes often exhibit characteristic colors, which arise from electronic transitions between d-orbitals of the metal ion. These colors can be used to identify the presence of specific metal ions in solution. For instance, the hexaaquacopper(II) ion is blue in color.

The stability of aquo complexes can vary widely. Factors affecting stability include the charge and size of the metal ion, the nature of the metal-ligand bond, and the presence of other ligands in the solution.

Examples

- Hexaaquairon(III) ion: \([\text{Fe(H}_2\text{O})_6]^{3+}\) - Hexaaquachromium(III) ion: \([\text{Cr(H}_2\text{O})_6]^{3+}\) - Hexaaquaaluminum(III) ion: \([\text{Al(H}_2\text{O})_6]^{3+}\)

Reactions

Aquo complexes can undergo various chemical reactions, including:

- Ligand exchange: Water molecules in the coordination sphere can be replaced by other ligands. - Acid-base reactions: Aquo complexes of metal ions with high charge density can act as acids, donating protons to form hydroxo complexes.

Also see

- Coordination complex - Ligand - Transition metal - Crystal field theory - Chelation

Coordination Chemistry
Main concepts
Coordination complex Ligand Coordination number Chelation Crystal field theory Ligand field theory Isomerism in coordination complexes
Types of ligands
Monodentate ligand Bidentate ligand Polydentate ligand Ambidentate ligand Bridging ligand
Applications
Bioinorganic chemistry Catalysis Medicinal chemistry Materials science
Notable complexes
Hemoglobin Chlorophyll Cisplatin Ferrocene
Related topics
Inorganic chemistry Organometallic chemistry Transition metal Metal complex