(a) Co-ordination compound

A co-ordination compound contains a central metal atom or cation which is surrounded by a suitable number of anions or neutral molecules and usually retains its identity in solutions as well as in solid state. To add a point further, in these compounds, the number of species surrounding the central metal atom is beyond its electrovalency or covalency. For example, [Cu(NH3)4]SO4, the electrovalency of copper is two, but it is surrounded by four NH₃ molecules. These ions or molecules (co-ordinated group or donor) attached to the central metal ion are called ligands.

A co-ordination compound generally contains one or more complex ions. For example,

[Cu(NH₃)₄]SO₄ contains [Cu(NH₃)₄]²⁺ complex ion

Na[Ag(CN)₂] contains [Ag(CN)₂]^- complex ion

[Ag(NH₃)₂]Cl contains [Ag(NH₃)₂]⁺ complex ion

[Cr(CN)₆] contains [Cr(CN)₆]^3- and 

[Co(NH₃)₆] [Co(NH₃)₆]³⁺ complex ion.

(b) Co-ordination entity (complex)

A complex ion is an electrically charged radical or species, carrying positive or negative charge, in which the central atom or ion is surrounded by a suitable number of neutral molecules or negative ions (called ligands). Some common examples are

[Ni(NH₃)₆]²⁺, [Co(NH₃)₄]²⁺, [PtCl₆]^2-, [Ag(CN₂)]^-, [Fe(CN)₆]⁴⁺, [Fe(C₂O₄)₃]^3-, [Ni(CO)₄], [Co(NH₃)₄Cl₂]

The complex ion carrying a positive charge is called cationic complex, the one with a negative charge is called anionic complex and the one with no charge is called a neutral complex.

(c) Ligands

The neutral molecules or ions that surround the metal ion in a complex are known as ligands (derived from the Latin word ligare meaning to bind). Ligands are normally polar molecules such as NH₃, H₂O or anions such as Cl^-, OH^-, Cn^- etc, which have at least one unshared pair of valence electrons. They get attached to the central metal ion by a co-ordinate or dative bond. The atom in the ligand which is involved in the formation of co-ordinate bond with central metal atom is called co-ordinating atom or donor atom. A donor atom may contain one or more unshared pair of electrons which are known as donor site(s) of the ligand. A ligand may contain one or more donor sites. Depending upon the number of donor sites, the ligands may be classified as:

unidentate : with one donor site.

di or bidentate: with two donor sites.

ter or tridentate : with three donor sites.

tetradentate: with four donor sites.

hexadentate: with six donor sites.

Ligands with more than one donor sites are collectively called multidentate ligands.

It may be noted that an atom can have only one donor site but a molecule can have more than one donor sites.