(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.