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Theme: Coordination compounds. Classification, structure, nomenclature, properties.
Transition metal ions characteristically form coordination compounds, which are usually colored and often paramagnetic. A coordination compound typically consists of a complex ion, a transition metal ion with its attached ligands, and counter ions, anions or cations as needed to produce a compound with no net charge.
Coordination compounds have been known since about 1700, but their true nature was not understood until the 1890s when a young Swiss chemist named Alfred Werner (1866–1919) proposed that transition metal ions have two types of valence (combining ability). One type of valence, which Werner called the secondary valence, refers to the ability of a metal ion to bind to Lewis bases (ligands) to form complex ions.
Structure of coordination compounds.
| K4[Fe(CN)6] – potassium hexacyanoferrate (II) | |
| K4[Fe(CN)6] | – External sphere |
| K4[Fe(CN)6] | – Internal sphere |
| K4[Fe(CN)6] | – Central atom |
| K4[Fe(CN)6] | – Coordinate relation |
| K4[Fe(CN)6] | – Ligand |
Central, complex forming, atoms is usually serves ions of metals of greater periods (Co, Ni, Pt, Hg, Ag, Cu). Typical ligands are OH-, CN-, NH3, CO, H2O; they connected with the central atom by donor-acceptor bound.
Coordination Number
The number of bonds formed by metal ions to ligands in complex ions varies from two to eight depending on the size, charge, and electron configuration of the transition metal ion. Many metal ions show more than one coordination number, and there is really no simple way to predict what the coordination number will be in a particular case.
Ligands
A ligand is a neutral molecule or ion having a lone electron pair that can be used to form a bond to a metal ion. The formation of a metal–ligand bond therefore can be described as the interaction between a Lewis base (the ligand) and a Lewis acid (the metal ion). The resulting bond is often called a coordinate covalent bond.
A ligand that can form one bond to a metal ion is called a monodentate ligand, or a unidentate ligand (from root words meaning “one tooth”).
Some ligands have more than one atom with a lone electron pair that can be used to bond to a metal ion. Such ligands are said to be chelating ligands, or chelates.
Preparation
Reactions of salts with ligands
AgCl + 2NH3 → [Ag(NH3)2]Cl
FeCl3 + 6KCN → K3[Fe(CN)6] + 3KCl
Chemical properties
1. Destruction of complexes at the expense of forming the slightly-soluble compounds
2[Cu(NH3)2]Cl + K2S → CuS↓ + 2KCl + 4NH3
2. The exchange of ligands between external and internal spheres
K2[CoCl4] + 6H2O → [Co(H2O)6]Cl2 + 2KCl
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