Types of ligand

Ligands are chemical species that co-ordinate with metal ions to form a complex. They are classified on the basis of the number of points of attachment to the central ion.

• Monodentate ligand - here the ligand is bound to the central ion at only one point, e.g. H₂O, NH₃.
• Bidentate ligand - this has two points of attachment to the central ion, e.g. ethylenediamine (en) (Fig. 23.1).
• Multidentate ligand - these have several points of attachment, e.g. ethylenediaminetetra-acetic acid (EDTA), which is a hexadentate ligand (six points of attachment) (Fig. 23.2).

Fig. 23.1 Structure of [Co(en)3]3+. It is a six co-ordinate octahedral complex of ethylenediamine (en) with cobalt (III).The complex has three five membered rings.

Fig.23.2 Structure of EDTA. (a) EDTA contains two donor N atoms and four donor 0 atoms. It can therefore form a hexadentate complex (b) with a metal ion, e.g. Pb2+.