# Coordination Compound Class 12th Notes

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Coordination Compound Class 12th Notes– Easy ncert notes and definitions of coordination compound

## Definition of Coordination Compounds

Those compounds which when dissolved in water or in any other solvent do not lost its identity and there properties are completely different form that of the constituent ions are known as coordination compounds. e.g. [CoCl3 (NH3)3] is a coordination compound in which cobalt is surrounded by three ammonia molecules and three chloride ions. The species within the square bracket are coordination entities or complexes and the ions outside the square bracket are called counter ions.

Definition of Complex ion- If the coordinate species carries a positive or negative charge, it is called a complex ion. E.g. [Cu(NH3)4]2+, [Fe(CN)6]4-.

Definition Double Salt– Those solid compounds which when dissolved in water undergo dissociation into its constituent ions means, they lose there identity in solution and possess the properties if individual ions, are known as double salt.

Werner’s Theory of Coordination Compounds– Alfred Werner a Swiss Chemist was the first to formulate his ideas about the structure of coordination compounds in 1898. He proposed the concept of a primary valence and a secondary valence for a metal ion.

• Primary Valencies– Those are non-directional and ionisable. These are satisfied by negative ions.
• Secondary Valencies– These are directional and non-inosable. This is equal to the number if ligands attached with the metal, i.e. equal to coordination number.

Definition of Ligand– Any atoms, molecules or ions which donate a pair of electrons to the central metal atom or ion and form coordinate bonds are called ligands. On the basis of charge, ligands can be positive, neutral or negative (which have at least one pair of electron which it can donate). E.g.

• Negative ligands${CN}^{-} , { NO }_{ 2 }^{ - }$
• Neutral ligands${H}_{2}{O}, {NH}_{3}$
• Positive ligands${ NO }_{ 2 }^{ + }, {NO}^{+} , { NO }_{ 4}^{ + }, etc$

#### Types if Ligands

1. Unidentate ligand– Those ligands which have only one pair of electrons available for donation, that can donate one pair of electrons to metal atom are known as monodentate or unidentate ligand.
2. Ambidentate Ligand– Unidentate ligands containing more than one coordinating atoms are called ambidentate ligands.
3. Bidentate ligand– Ligands having two pairs of electrons to donate, which it donate to the metal atom are known as bidentate ligand.
4. Chelate ligand– Multidentate ligand when combine with the central metal ion at two or more than two binding sites they form a ring structure which is known as chelate ligand.
5. Polydentate ligand– Those ligands which have two or more than two binding sites which it can donate electron pair to the central metal atom are known as multidentate ligand. They get attached with the central metal atom at more than one site.

Definition of Coordination Number– The number of ligand donor atoms to which the metal is directly bonded is called coordination number.

Definition of Coordination Sphere- The central metal atom or ion bonded with the ligand are enclosed in square bracket and are collectively called coordination sphere. e.g. In the complex K4[Fe(CN)6], [Fe(CN)6]4- is known as coordination sphere.

Oxidation Number of Central Metal Atom– If the groups present in the complex, apart from the central metal atom are removed along with there shared electron pair, then the charge present on the central ion is the oxidation number of central metal atom.

Definition of Homoleptic and Heteroleptic Complexes– Complexes in which a metal is bound to only one type of donor groups are known as homoleptic, e.g. [Co(NH3)6]3+. Complexes in which a metal is bound to more than 1 kind of donor groups are called as heteroleptic.

Charge on Coordination Compound– Charge on a coordination compound is algebraic sum of charge on coordinated group, central atom and ligand.

Isomerism in Coordination Compounds- Those compounds which have same molecular formula but different structural formula or orientation of atoms in space are known as isomers and the process is known as isomerisation.

This are of two types

Structural isomerism- The coordination compound having same chemical formulas, but different structural arrangement is known as structural isomers. This are of two types-

• Ionisation Isomerism- When the complexes with same composition give different ions in solution. e.g. [Co(NH3)5Br]SO4 and [Co(NH3)5SO4] Br.
• Linkage isomerism- When complexes differ only in the point of attachment of the ambidentate ligand.
• Solvent/Hydrate Isomerism- When complexes differ in the number of water (Solvent) molecules as ligand.
• Coordination Isomerism- When cation and anion both are complex ions and differ in distribution of ligands.

Stereoisomers– The coordination compounds having same molecular as well as structural formulae but different arrangement is space in the polyhedral are called steroisomers.        It is 2 types-

• Geometrical Isomerism– This arises when the atoms or ligands bonded directly to the central metal ion have different spatial arrangement. This type of isomerism is possible only in heterolytic complexes. Geometrical isomeriesm is of two types cis and trans. In cis-isomerism, two X ligands (unidentate ligands) are at adjecent position. Whereas in trans-isomerism, two X ligands are at a position opposite to each other.
• Optical Isomerism– Coordination compounds which are mirror images of each other and have a chiral centre show optical isomerism. This generally shown by octahedral complexes having bidentate ligands.

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