Question

Which of the following ligands forms a chelate complex ?

• A. Ammonia
• B. Water
• C. \(NO_2^{-}\)
• D. Oxalate ion

Hint:

Common chelating ligands: \[ C_2O_4^{2-},\; en,\; EDTA^{4-} \] Oxalate ion is a bidentate ligand and readily forms chelate complexes.

Answer 1

The Correct Option is D

Concept: A chelate complex is formed when a multidentate ligand coordinates to the central metal atom through two or more donor atoms simultaneously, thereby forming one or more ring structures. Ligands can be classified as:

• Monodentate ligands : Coordinate through one donor atom.

• Bidentate ligands : Coordinate through two donor atoms.

• Polydentate ligands : Coordinate through more than two donor atoms.
Chelation occurs only when the ligand possesses more than one donor atom capable of bonding to the metal ion at the same time.

Step 1: Examining ammonia as a ligand. Ammonia \((NH_3)\) donates a lone pair through a single nitrogen atom. Hence it is a monodentate ligand. Since it can attach through only one donor atom, it cannot form a chelate ring. Therefore, Option (A) is incorrect.

Step 2: Examining water as a ligand. Water \((H_2O)\) coordinates through only one oxygen atom. Therefore it is also a monodentate ligand and cannot form a chelate complex. Hence, Option (B) is incorrect.

Step 3: Examining nitrite ion as a ligand. Nitrite ion \((NO_2^-)\) is an ambidentate ligand. It can coordinate through either nitrogen or oxygen, but not simultaneously through both atoms to form a chelate ring. Hence it does not normally act as a chelating ligand. Therefore, Option (C) is incorrect.

Step 4: Examining oxalate ion as a ligand. Oxalate ion is represented as \[ C_2O_4^{2-} \] It contains two oxygen donor atoms capable of coordinating simultaneously to the same metal ion. Therefore, oxalate acts as a bidentate ligand. It forms a ring structure with the metal ion and hence produces a chelate complex. Therefore, Option (D) is correct. \[ \boxed{\text{Option (D)}} \]