Choose the incorrect statement about ethylenediaminetetraacetate ion:

It is an ambidentate ligand

The correct answer is option 4. It is an ambidentate ligand.

Let us break down the details for each statement about the ethylenediaminetetraacetate ion (EDTA) to understand why statement 4 is incorrect.

Statement 1: "It is a hexadentate ligand."

Hexadentate ligand means that the ligand has six donor atoms capable of binding to a central metal ion. EDTA has six donor atoms: four oxygen atoms from the carboxylate groups \((-COO^-)\) and two nitrogen atoms from the amine groups \((-NH_2)\). These six atoms can coordinate with a metal ion simultaneously, making EDTA a hexadentate ligand.

Statement 2: "It forms chelate when binds with a metal ion."

A chelate is a complex formed when a ligand binds to a metal ion at multiple sites, creating a ring-like structure. EDTA can form multiple bonds with a metal ion, resulting in a stable ring structure known as a chelate. This chelating effect is due to the multiple points of attachment (the two nitrogen and four oxygen atoms) to the metal ion. Chelation increases the stability of the metal-ligand complex, which is why EDTA is commonly used as a chelating agent in various applications.

Statement 3: "It can bind through two nitrogen and four oxygen atoms."

EDTA has two nitrogen atoms from the amine groups and four oxygen atoms from the carboxylate groups, all of which can coordinate with a metal ion. When EDTA binds to a metal ion, these six atoms coordinate simultaneously, making EDTA a hexadentate ligand. This ability to bind through these specific atoms is crucial for its role as a chelating agent.

Statement 4: "It is an ambidentate ligand." (Incorrect Statement)

Ambidentate ligands are ligands that have two possible donor atoms, but only one of these atoms can coordinate with the metal ion at a time. Examples of ambidentate ligands include thiocyanate ion \((SCN^-)\), which can bind through either sulfur (S) or nitrogen (N), but not both simultaneously. EDTA, however, is not ambidentate. It does not have alternative donor atoms that could individually bind to the metal ion. Instead, EDTA uses all six donor atoms (four oxygens and two nitrogens) to bind to the metal ion simultaneously. Therefore, EDTA is a polydentate (specifically hexadentate) ligand, not an ambidentate ligand. This makes **statement 4 incorrect** because it mischaracterizes the nature of EDTA's binding ability.

Conclusion:

EDTA is a hexadentate ligand, forming stable chelates with metal ions by coordinating through two nitrogen and four oxygen atoms. It is not an ambidentate ligand, as it does not have two alternative donor atoms that can individually bind to the metal ion; instead, it binds through all six donor atoms simultaneously. Thus, the incorrect statement is statement 4.