Read the Passage carefully and answer the questions.
When a ligand is bound to a metal ion through a single donor atom, it is said to be a unidentate ligand but when it bind through two donor atoms, then it is said to be didentate ligand and when several donor atoms are present in a single ligand, it is said to be a polydentate ligand. When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. ligand which has two different donor atoms and either of the two ligetes in the complex is called ambidentate ligands. The number of ligand donor atoms to which the metal is directly bonded, is called coordination number of the metal ion in the complex. Complexes in which a metal is bound to only one kind of donor groups are known as homoleptic and in which a metal is bound to more than one kind of donor groups are known as heteroleptic complexes.
To make an octahedral complex of \( Mg^{2+} \) ion, how many molecules of EDTA are used?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → one
The number of donor atoms through which one ligand binds is its denticity. Ethylenediaminetetraacetate, \( \ce{EDTA^4-} \), is a hexadentate ligand: it binds through two nitrogen atoms and four oxygen atoms (of its four carboxylate groups) at the same time, wrapping round the metal ion as a chelate.
An octahedral complex has six donor atoms around the metal ion. One EDTA ion supplies all six:
\( \displaystyle \ce{Mg^2+ + EDTA^4- -> [Mg(EDTA)]^2-} \)
\( \displaystyle \text{donor atoms} = 1 \times 6 = 6 \)
So only one molecule of EDTA is needed to make the octahedral complex of \( \ce{Mg^2+} \).
Why the other options are incorrect
Option (2) six — Incorrect. Six would be the number of molecules needed for a unidentate ligand such as \( \ce{NH3} \) or \( \ce{H2O} \); six EDTA ions would offer 36 donor atoms.
Option (3) three — Incorrect. Three ligands complete an octahedron only when each is didentate, as en or oxalate in \( \ce{[Co(en)3]^3+} \).
Option (4) two — Incorrect. Two EDTA ions would give 12 donor atoms, far more than the six positions of an octahedron.
NCERT (2026-27 edition):
Ligands – …
