Consider the two complex ions \([CoF_6]^{3-}\) and \([Co(C_2O_4)_3]^{3-}\), which of the following statement (s) is/are false :
(A) Both are octahedral.
(B) \([Co(C_2O_4)_3]^{3-}\) is diamagnetic, while \([CoF_6]^{3-}\) is paramagnetic.
(C) Both are outer orbital complexes.
(D) In both the complexes the central metal is in the same oxidation state.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) Only
In both ions cobalt is Co3+: in \( \ce{[CoF6]^3-} \), x + 6(−1) = −3 gives x = +3; in \( \ce{[Co(C2O4)3]^3-} \), x + 3(−2) = −3 gives x = +3. Co (Z = 27) is [Ar] 3d7 4s2; losing the two 4s electrons and one 3d electron gives \( \ce{Co^3+} \): [Ar] 3d6. Valence bond theory then tells the two ions apart:
\( \ce{[CoF6]^3-} \): \( \ce{F^-} \) is a weak field ligand, so the six 3d electrons stay as ↑↓ ↑ ↑ ↑ ↑ (4 unpaired). No 3d orbital is free, so \( \ce{Co^3+} \) uses 4s, 4p and 4d orbitals: sp3d2, outer orbital, paramagnetic.
\( \ce{[Co(C2O4)3]^3-} \): oxalate pairs the six electrons into three 3d orbitals, ↑↓ ↑↓ ↑↓ (0 unpaired), leaving two 3d orbitals free: d2sp3, inner orbital, diamagnetic.
(A) Both are octahedral. — True. Both have coordination number 6: six \( \ce{F^-} \) ions, or three didentate oxalate ions. Six-coordinate sp3d2 and d2sp3 hybrids are both octahedral.
(B) \([Co(C_2O_4)_3]^{3-}\) is diamagnetic, while \([CoF_6]^{3-}\) is paramagnetic. — True. In the oxalato complex all six 3d electrons of \( \ce{Co^3+} \) are paired (0 unpaired), so it is diamagnetic; in the fluoro complex 4 electrons stay unpaired, so it is paramagnetic.
(C) Both are outer orbital complexes. — False. Only \( \ce{[CoF6]^3-} \) uses the outer 4d orbitals (sp3d2); \( \ce{[Co(C2O4)3]^3-} \) uses the inner 3d orbitals (d2sp3), so it is an inner orbital complex.
(D) In both the complexes the central metal is in the same oxidation state. — True. Cobalt is +3 in both, as worked out above.
NCERT (2026-27 edition):
Magnetic Properties of Coordination Compounds – “… The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing d6 ions. However, with species containing d4 and d5 ions there are complications. [Mn(CN)6]3– has magnetic moment of two unpaired electrons while [MnCl6]3– has a paramagnetic moment of four unpaired electrons. [Fe(CN)6]3– has magnetic moment of a single unpaired electron while [FeF6]3– has a paramagnetic moment of five unpaired electrons. [CoF6]3– is paramagnetic with four unpaired electrons while [Co(C2O4)3]3– is diamagnetic. This apparent anomaly is explained by valence bond theory in terms of formation of inner orbital and outer orbital coordination entities. [Mn(CN)6]3–, [Fe(CN)6]3– and [Co(C2O4)3]3– are inner orbital complexes involving d2sp3 hybridisation, the former two complexes are paramagnetic and the latter diamagnetic. On the other hand, [MnCl6]3–, [FeF6]3– and [CoF6]3– are outer orbital complexes involving sp3d2 hybridisation and are paramagnetic corresponding to four, five and four unpaired electrons.”