Question

Which one of the following is an outer orbital complex?

• A. $[{Co}({NH}_3)_6]^{3+}$
• B. $[{Fe}({CN})_6]^{3-}$
• C. $[{CoF}_6]^{3-}$
• D. $[{Co}({C}_2{O}_4)_3]^{3-}$
• E. $[{Fe}({NH}_3)_6]^{3+}$

Hint:

Weak field ligands like fluoride favor high-spin, outer orbital complexes, whereas strong field ligands induce low-spin, inner orbital configurations.

Answer 1

The Correct Option is C

Step 1: Inner orbital complexes use low-spin configurations with $d^2sp^3$ hybridization, whereas outer orbital complexes use high-spin configurations with $sp^3d^2$ hybridization. 
Step 2: Fluoride ($F^-$) is a weak field ligand and does not cause strong crystal field splitting. As a result, cobalt in $[{CoF}_6]^{3-}$ adopts an outer orbital configuration with $sp^3d^2$ hybridization. 
Step 3: The other complexes: 
$[{Co}({NH}_3)_6]^{3+}$ and $[{Fe}({CN})_6]^{3-}$ involve strong field ligands, leading to inner orbital configurations. 
$[{Co}({C}_2{O}_4)_3]^{3-}$ also forms an inner orbital complex. 
Step 4: Since $[{CoF}_6]^{3-}$ is an outer orbital complex, the correct answer is (C).