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Q. Consider the following two complex ions: $[CoF_6]^{3-}$ and $[Co(C_2O_4)_3]^{3-}$ . Which of the following statement (s) is/are false?
(I) Both are octahedra l
(II) $[Co(C_2O_4)_3]^{3-}$ is diamagnetic while $[CoF_6]^{3-}$ is paramagnetic
(III) Both are outer orbital complexes
(IV) In both the complexes the central metal is in the same oxidation state

KEAMKEAM 2016Coordination Compounds

Solution:

All statements are true except (III), that both are outer orbital complexes. In fact, In both the given complexes, the central metal is in the same oxidation state, i.e. $+3$

$\left[ CoF _{6}\right]^{3-}$ due to weak ligand (F) does not go for pairing and show outer octahedral orbital complex $\left(s p^{3} d^{2}\right) .$ As it has five unpaired electrons, therefore, it is paramagnetic in nature.

While $\left[ Co \left( C _{2} O _{4}\right)_{3}\right]^{3-}$ due to strong bidentate ligand, go for pairing and forms inner octahedral orbital complex $\left(d^{2} s p^{3}\right)$. As it does not have unpaired electrons, therefore it is diamagnetic in nature.