IUPAC name of the isomer of compound with molecular formula, \(FeCl_3.6H_2O\) having \(i = 3\) is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Pentaaquachlorido iron (III) chloride monohydrate
\( \ce{FeCl3.6H2O} \) can exist as hydrate (solvate) isomers, like \( \ce{CrCl3.6H2O} \). \( \ce{Fe^3+} \) has coordination number 6; the isomers differ in how many water molecules are inside the coordination sphere and how many \( \ce{Cl-} \) ions are outside it. The van’t Hoff factor i counts the ions produced in solution:
\( \displaystyle \ce{[Fe(H2O)6]Cl3 -> [Fe(H2O)6]^3+ + 3Cl-}\qquad i = 4 \)
\( \displaystyle \ce{[Fe(H2O)5Cl]Cl2.H2O -> [Fe(H2O)5Cl]^2+ + 2Cl- + H2O}\qquad i = 3 \)
\( \displaystyle \ce{[Fe(H2O)4Cl2]Cl.2H2O -> [Fe(H2O)4Cl2]+ + Cl- + 2H2O}\qquad i = 2 \)
\( \displaystyle \ce{[Fe(H2O)3Cl3].3H2O}\ \text{gives no ions}\qquad i = 1 \)
The isomer with i = 3 is \( \ce{[Fe(H2O)5Cl]Cl2.H2O} \): five aqua ligands and one chlorido ligand (coordination number 6), two chloride counter ions and one water of crystallisation. Its name is pentaaquachloridoiron(III) chloride monohydrate.
Why the other options are incorrect
Option (1) Tetraaquadichlorido iron (III) chloride dihydrate — Incorrect. \( \ce{[Fe(H2O)4Cl2]Cl} \)·\( \ce{2H2O} \) gives only two ions, i = 2.
Option (2) Triaquatrichlorido iron (III) — Incorrect. \( \ce{[Fe(H2O)3Cl3]} \) is a neutral complex, i = 1; it also accounts for only three of the six water molecules.
Option (3) Tetraaquachlorido iron (III) dichloride dihydrate — Incorrect. “tetraaquachlorido” puts only five ligands around \( \ce{Fe^{3+}} \), but its coordination number is 6; with five aqua ligands inside the sphere the same composition is named pentaaquachlorido, Option (4).
NCERT (2026-27 edition):
Solvate Isomerism – “This form of isomerism is known as ‘hydrate isomerism’ in case where water is involved as a solvent. This is similar to ionisation isomerism. Solvate isomers differ by whether or not a solvent molecule is directly bonded to the metal ion or merely present as free solvent molecules in the crystal lattice. An example is provided by the aqua complex [Cr(H2O)6]Cl3 (violet) and its solvate isomer [Cr(H2O)5Cl]Cl2.H2O (grey-green).”