Question

In the brown ring test, the brown complex formed :

• A. has a formal +1 oxidation state on $Fe$
• B. has +2 charge on $Fe$
• C. shows a magnetic moment of 3.9 BM
• D. has the brown colour due to charge transfer from $NO$ to $Fe$

Answer 1

Answer: (A), (C), (D)

The correct options are
A has a formal +1 oxidation state on $Fe$
C shows a magnetic moment of 3.9 BM
D has the brown colour due to charge transfer from $NO$ to $Fe$

The brown ring complex is $\left[Fe\right(H_{2}O{)}_5\left(NO\right)]S{O}_4$.

The overall charge on the complex ion is +2. Out of this, $NO$ has a formal charge of +1. Hence, $Fe$ has a formal charge of +1.

Iron has $3d^7$ as the outer electronic configuration. It undergoes $s{p}^3{d}^2$ hybridization.

Iron contains 3 unpaired electrons and has a magnetic moment of 3.9 B.M.
$\sqrt{n(n+2)}=\sqrt{3(3+2)}=3.9B.M$.

Charge transfer from $NO$ ligand to central iron metal results in a brown colour.

Hence the correct options are A, C and D.