How many EDTA (ethylenediaminetetraacetic acid) molecules are required to make an octahedral complex with a Ca2+ ion?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → One
The denticity of a ligand is the number of donor atoms through which one ligand binds to the metal ion. An octahedral complex has six coordination positions around the metal ion.
Donor atoms in EDTA: the ion \( \ce{EDTA^4-} \) binds through 2 nitrogen atoms and 4 oxygen atoms, so it is a hexadentate ligand (6 donor atoms).
Positions to fill: 6 in an octahedral complex.
EDTA needed: the six donor atoms of a single ligand match the six positions, so one EDTA wraps round the \( \ce{Ca^2+} \) ion and fills all six positions, giving \( \ce{[Ca(EDTA)]^2-} \).
NCERT (2026-27 edition):
5.2 Definitions of Some Important Terms Pertaining to Coordination Compounds – “… Ethylenediaminetetraacetate ion (EDTA4–) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.”
5.7 Importance and Applications of Coordination Compounds – “The Ca2+ and Mg2+ ions form stable complexes with EDTA.”