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Thursday, 23 February 2017

Co-ordination Compounds - I


Co-ordination compounds:

A coordination compound contains a central metal atom or ion surrounded by number of oppositely charged ions or neutral molecules. These ions or molecules re bonded to the metal atom or ion by a coordinate bond.

Example: K4[Fe(CN)6]
They do not dissociate into simple ions when dissolved in water.

Double salt
When two salts in stoichiometric ratio are crystallised together from their saturated solution they are called double salts
Example:FeSO4.(NH4)2SO4.6H 2O (Mohr’s salt)
They dissociate into simple ions when dissolved in water.

Coordination entity:
A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.
Example: In K4[Fe(CN)6], [Fe(CN)6] 4- represents coordination entity.

Central atom or ion:
In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.
Example: In K4[Fe(CN)6], Fe²+ is the central metal ion.

Ligands:
A molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is called ligand.
It may be neutral, positively or negatively charged.
Examples: H2O, CN  -,NO +etc.

Donor atom:
An atom of the ligand attached directly to the metal is called the donor atom.
Example: In the complex K4[Fe(CN)6], CN is a donor atom.

Coordination number:
The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.
Example: In the complex K4[Fe(CN)6], the coordination number of Fe is 6.

Coordination sphere:
The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the coordination sphere.
Example: In the complex K4[Fe(CN)6], [Fe(CN)6]4- is the coordination sphere.

Counter ions:
The ions present outside the coordination sphere are called counter ions.
Example: In the complex K4[Fe(CN)6], K+ is the counter ion.

Coordination polyhedron:
The spatial arrangement of the ligand atoms which are directly attached to the central atom/ ion defines a coordination polyhedron about the central atom.
The most common coordination polyhedra are octahedral, square planar and tetrahedral.
Examples: [PtCl 4]2- is square planar, Ni(CO)is tetrahedral while [Cu(NH3)6]3+ is octahedral.

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