Question

Which of the following is an outer orbital complex?

• A. $[Fe(CN)_6]^{4-}$
• B. $[FeF_6]^{3-}$
• C. $[Co(NH_3)_6]^{3+}$
• D. $[Co(CN)_6]^{3-}$

Answer 1

Answer: (B)

$[FeF_6]^{3-}$

Answer 2

In $[FeF_6]^{4-}$ and $[Fe(CN)_6]^{4-}$, iron is in $+2$ oxidation state.

$F^-$ is a weak ligand, hence $3d$ electrons do not pair up. As a result, $[FeF_6]^{4-}$ has four unpaired electrons and it forms outer orbital complex ($sp ^3d^2$-hybridisation). Whereas, $CN^-$ is a strong ligand, hence $3d$ electrons
pair up. As a result, there is no unpaired electrons and it form inner orbital complex ($d^2sp^3$ hybridisation).
In $[CoCNH_3)_6]^{3+}$ and $[Co(CN)_6]^{3-}$, cobalt is in $+3$ oxidation state.

$NH_3$ and $CN^-$, both are strong held ligand, hence $3d$ electrons pair up and both form inner orbital complex ($d^2sp^3$ hybridisation).