How many EDTA (ethylenediaminetetraacetic acid) molecules are required to make an octahedral complex with a $$Ca^{2+}$$ ion?

An octahedral complex has a coordination number $$6$$; the central metal ion must be bonded to six donor atoms.

EDTA$$^{4-}$$ (ethylenediaminetetraacetate) is a hexadentate ligand. It possesses six separate donor atoms: two nitrogen atoms from $$-NH_2$$ groups and four oxygen atoms from deprotonated $$-COO^-$$ groups. Therefore, a single EDTA $$^{4-}$$ ligand can coordinate through all six sites simultaneously.

When a $$Ca^{2+}$$ ion binds one EDTA$$^{4-}$$ ligand, all six coordination positions around calcium are occupied, producing the chelate $$[Ca(EDTA)]^{2-}$$ with octahedral geometry.

Hence, only one molecule of EDTA is required.

Option C which is: one