Question

Which among the following complexes is more stable?

• A. ${\left[Fe{\left(CN\right)}_6\right]}^{4-}$
• B. ${\left[Fe{\left(Cl\right)}_6\right]}^{3-}$
• C. ${\left[Fe{\left(CN\right)}_6\right]}^{3-}$
• D. ${\left[Fe\left(C_2{O}_4\right)\right]}^{3-}$

Answer 1

The correct option is A ${\left[Fe{\left(CN\right)}_6\right]}^{4-}$

Since $F{e}^{2+}\left(d^6\right)$ is presence of strong field ligand like $C{N}^{-}$ has more CFSE $(2/5\times 6{△}_o)$ than $F{e}^{3+}\left(d^5\right)$ having less CFSE $\left(-2/5\times \triangle_o\right) than $f{e}^{2+}$, so $A$ is more stable and $C{N}^{-}$ is more stronger than $C_2{O}_4^{2-}$ in ligand field strength.