Total number of isomers for the complex \([FeCl_2(C_2O_4)_2]^{4-}\) is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3
\( \ce{[FeCl2(C2O4)2]^{4-}} \) is an octahedral complex of the type [M(AA)2B2]: two didentate oxalate ligands (AA) and two unidentate chloride ligands (B). Oxidation state of Fe: x + 2(−1) + 2(−2) = −4, so x = +2.
trans isomer — the two \( \ce{Cl^{-}} \) ions are opposite each other (180°). The two oxalate rings lie in one plane, so the molecule has a mirror plane and is optically inactive: 1 isomer.
cis isomer — the two \( \ce{Cl^{-}} \) ions are adjacent (90°). The two chelate rings are then arranged like a propeller with no plane of symmetry, so the cis form is chiral and exists as a pair of enantiomers (d and l): 2 isomers.
Total stereoisomers = 1 (trans) + 2 (cis d and l) = 3. This is the same pattern as cis- and trans-\( \ce{[PtCl2(en)2]^{2+}} \), where only the cis form is optically active.
NCERT (2026-27 edition):
Optical Isomerism – “Optical isomerism is common in octahedral complexes involving didentate ligands (Fig. 5.6). In a coordination entity of the type [PtCl2(en)2]2+, only the cis-isomer shows optical activity (Fig. 5.7).”
