Central metal atom & ligand are the part of coordination compound. Before going to the topic we should know that what is coordination complex.
Coordination complex consist of a central metal atom & surrounding which many ions are joined by coordinate bond called as ligand. They form a coordination sphere.
CENTRAL METAL ATOM
Central metal atom acts as Lewis Acid ( accept a lone pair of electron ).It contains vacant orbitals. It accepts lone pair of electron from the ligand due to availability of vacant orbitals.
These are the ions which donate lone pair of electron to the central metal atom & act as Lewis base. These are classified into different types by their donating sites.
Unidentate ligand – Ligand which has one donor atom. EX – NH3, Cl–, F– etc.
Bidentate ligand – Ligand which has two donor atoms. EX – ethylenediamine, C2O42-(oxalate ion) etc.
Tridentate ligand – Ligand which has three donor atoms. EX – e.g. (dien) diethyl triamine.
Hexadentate ligand – Ligand which has six donor atoms. EX – EDTA.
Chelating ligand – This is the multidentate ligand which simultaneously donating electron to the metal atom through more than one site. These ligands produce a ring like structure called chelate. Chelation increases the stability of complex. This effect is called chelation effect.