Question

When SCN\(^{-}\) is added to an aqueous solution containing Fe(NO\(_3\))\(_3\), the complex produced is

• A. \([Fe(H_2O)_2(SCN^-) ]^{2+}\)
• B. \([Fe(H_2O)_5(SCN^-) ]^{2+}\)
• C. \([Fe(H_2O)_8(SCN^-) ]^{2+}\)
• D. \([Fe(H_2O)(SCN^-) ]^{6+}\)

Hint:

Fe\(^{3+}\) with SCN\(^{-}\) forms blood-red complex \([Fe(SCN)]^{2+}\) in aqueous medium.

Answer 1

The Correct Option is B

Step 1: Identify initial complex of Fe\(^{3+}\) in water.
Fe\(^{3+}\) in aqueous medium exists mainly as:
\[ [Fe(H_2O)_6]^{3+} \]
Step 2: Addition of thiocyanate ion.
SCN\(^{-}\) replaces one water molecule and forms a red coloured complex.
\[ [Fe(H_2O)_6]^{3+} + SCN^- \rightarrow [Fe(H_2O)_5(SCN)]^{2+} + H_2O \]
Step 3: Charge calculation.
Fe\(^{3+}\) + 5 neutral water + SCN\(^{-}\) gives overall charge:
\[ +3 + 0 -1 = +2 \]
Step 4: Final conclusion.
The complex formed is \([Fe(H_2O)_5(SCN)]^{2+}\).
Final Answer:
\[ \boxed{[Fe(H_2O)_5(SCN)]^{2+}} \]