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|Important Terms Involving Coordination Compounds|
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Coordination chemistry is the branch of inorganic chemistry that concerns the study of coordination compounds.
Coordination compounds are chemical compounds that consist of an array of anions or neutral molecules that are bound to a central atom via coordinate covalent bonds. Coordination compounds are also referred to as coordination complexes.
The definitions of some important terms in the chemistry of coordination compounds can be found below.
A chemical compound in which the central ion or atom (or the coordination centre) is bound to a set number of atoms, molecules, or ions is called a coordination entity.
Some examples of such coordination entities include [CoCl3(NH3)3], and [Fe(CN)6]4-.
As discussed earlier, the atoms and ions to which a set number of atoms, molecules, or ions are bound are referred to as the central atoms and the central ions.
In coordination compounds, the central atoms or ions are typically Lewis Acids and can, therefore, act as electron-pair acceptors.
The coordination number of the central atom in the coordination compound refers to the total number of sigma bonds through which the ligands are bound to the coordination centre.
For example, in the coordination complex given by [Ni(NH3)4]2+, the coordination number of nickel is 4.
The non-ionizable part of a complex compound which consists of central transition metal ion surrounded by neighboring atoms or groups enclosed in square bracket.
The coordination centre, the ligands attached to the coordination centre, and the net charge of the chemical compound as a whole, form the coordination sphere when written together.
This coordination sphere is usually accompanied by a counter ion (the ionizable groups that attach to charged coordination complexes).
Example: [Co(NH3)6]C/3 – coordination sphere
The geometric shape formed by the attachment of the ligands to the coordination centre is called the coordination polyhedron.
Examples of such spatial arrangements in coordination compounds include tetrahedral and square planar.
The oxidation number of the central atom can be calculated by finding the charge associated with it when all the electron pairs that are donated by the ligands are removed from it.
For example, the oxidation number of the platinum atom in the complex [PtCl6]2- is +4.
Double salts are completely ionizable in aqueous solutions and each ion in the solution gives the corresponding confirmatory test.
Example: Potash Alum is double sulphate. It is K2SO4.Al2(SO4)3.24H2O on Ionization it gives:
K+, SO2−4 and Al+3 ions which response to the corresponding tests.
Ionization of Mohr's Salt
Co-ordinate complex are incompletely ionizable in the aqueous solutions. These give a complexion which does not show complete ionization.
Example: Potassium Ferrocyanide. [K4Fe(CN)6] It ionizes to give K+ and [Fe(CN)6]−4 [ferro cyanide ions]
The atoms, molecules, or ions that are bound to the coordination centre or the central atom/ion are referred to as ligands.