Monodentate ligands are Lewis bases that donate a single pair ("mono") of electrons to a metal atom. Monodentate ligands can be either ions (usually anions) or neutral molecules.

Chemists often represent ligands as spheres for simplicity, even though the "sphere" sometimes has three-dimensional structure of its own. For example, when chemists draw the structure for [Ni(NH₃)₆]²⁺, each ammonia ligand is represented as a sphere. The sphere represents the donor atom of the ligand. In [Ni(NH₃)₆]²⁺, the donor atoms are the nitrogen atoms of the NH₃ ligands (NOT the hydrogen atoms).

Some ligands can bond to a metal atom using more than two pairs of electrons. An example is ethylenediamminetetraacetate ion (EDTA^4-), the Lewis structure of which is shown below. EDTA^4- forms very stable complexes with most of the transition metals.

Bidentate ligands are Lewis bases that donate two pairs ("bi") of electrons to a metal atom.

Bidentate ligands are often referred to as chelating ligands ("chelate" is derived from the Greek word for "claw") because they can "grab" a metal atom in two places.

A complex that contains a chelating ligand is called a chelate.