Correct Answer - A
A monodenate ligand (meaning ' one - toothed' ligand) is a ligand that bonds to a metal atom/ion through one atom of the ligand. In some complexes a ligand occupies more than one coordination position. Thus more than one atom in the ligand is bonded to the central metal. For example, ethylinediamine (en) forms a complex with coppeer ions:
In this complex the copper is surrounded by four `-NH_(2)` group. Thus each ethylenedimine molecule is bonded to the `cu^(2+)` in two place. For this reason ethylendiamine is called a bidentate ligand (Bidentate means literally two teeth !) Thus, a ring structure is formed (in this case a pair of five-membered rings). Such rings structure are called chelates and the corresponding ligands are called chelating ligands. All chelating are polydentate ligands. A polydentate ligand ('having many teeth') is a ligand that can bond with two or more atoms to a metal atom. A complex formed by polydentate ligands is frequently quite stable and is called a chelate. Chelated complexes are more stable than similar complexes with unidentate lengands, as dissociation of the omplex invlves breaking two bonds rather than one. Chelates are even more stable when they contian a system of alternate double and single bonds. This is better represented as a system in which electron density is delocalized and spread over the ring. Examples of this include acetylcetone and porphyrin complexes with metals.
Resonance in acetylacetone chelate
