Friday, October 31, 2008

Coordination Compounds - Important Terms - Revision Points

Coordination compounds: coordination compounds are a special class of compounds in which the central atom is surrounded by ions and atoms beyond their normal valency


Coordination or complex compounds may be defined as a molecular compound that results from the combination of two or more simple stable molecular compounds and retains its identity in the solid as well as in the dissolved state .The properties of such compounds are totally different than individual constituents. A coordination compound contains very often but not always a complex ion.

Complex ion
An electrically charged species which consists of a central metal ion or atom surrounded by a group of ions or neutral molecules(it may be noted that a complex may be positively charged or negatively charged or neutral).
Types of complex:
1.cationic complex
2.anionic complex
3.neutral complex
cationic complex :A complex which has net positive charge .
anionic complex : A complex which has net negative charge .
neutral complex: A complex which has no net charge or simply neutral.


1 .Ligands:
The neutral molecule or ions which are directly attached to the central metal ion or atom through coordinate bonds in the complex ion is called Ligand.
>Ligands should have lone pair of electrons
>Ligands donate the lone pair to the central metal atom or ion forming coordinate covalent bond .
>thus Ligand are Lewis bases and central metal ion is a Lewis acid.

Types of ligands:
1. monodentate or unidentate ligands :ligand having only one donor atom.
2.bidentate ligand :ligand having two donor atoms
3.polydentate ligand :ligand having more than two donor atoms.

Chelating ligands:
when a bidentate or a polydentate ligand is attached by two or more donor atoms to the same central metal ion or atom forming a ring structure ,the ligand is called a chelating ligand. Chelating ligand forms a ring structure around the central metal ion.

Ambident ligand:
The monodentate ligands which can coordinate with the central metal ion or atom through more than one site are called ambident ligand.e.g.CN

M CN (cyanide)
M NC(isocyanide)

2.Coordination number:
The total number of ligands attached to the central metal ion or atom is called the coordination number of the metal atom or ion.

3.Coordination sphere:
The central metal ion or atom and the ligands are collectively called the coordination sphere .
(in other words the ions present in the square bracket together are called the coordination sphere)

4. Charge of a complex:
the charge carried by a complex ion is the algebraic sum of the charges carried by the central ion and the ligands coordinated to it .

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