Question

$[Fe(CN)_{6}]^{4-}$ and $[Fe(H_{2}O )_{ 6}]^{2+}$ show different colours in dilute solution because

• A. $CN^{-}$ is a strong field ligand and $H_{2}O$ is a weak field ligand hence magnitude of $CFSE$ is different
• B. both $CN^{-}$ and $H_{2}O$ absorb same wavelength of energy
• C. complexes of weak field ligands are generally colourless
• D. the sizes of $CN^{-}$ and $H_{2}O$ are different hence their colours are also different

Answer 1

Answer: (A)

$\left.Fe ( CN )_{6}\right]^{4-} \Rightarrow Fe ^{2+} \Rightarrow [ Ar ] 3 d ^{6}$
Where $Fe ^{2+}= CN \rightarrow$ Strong field ligand.
So, the energy gap is high.
$\left.Fe \left( H _{2} O \right)_{6}\right]^{2+} \Rightarrow Fe ^{2+} \Rightarrow [ Ar ] 3 d ^{6}$
Where $H _{2} O =$ Weak field ligand.
Energy gap is low.