Identifying Complex P, Q, and R

The question asks us to identify three iron complexes, P, Q, and R, based on their colors and reactions with specific reagents. We are given the color of P (yellow), the color of Q (pale violet), and the color of R (yellow), along with the reactions that transform P into Q and Q into R.

Complex P: The Starting Yellow Complex

Complex P is described as yellow. We need to look at the options and see which complex is likely to be yellow. The first option suggests P is \( \text{[Fe(H}_2\text{O)}_5\text{(OH)]}^{2+} \). Iron(III) hydrolysis products, like \( \text{[Fe(H}_2\text{O)}_5\text{(OH)]}^{2+} \), are often yellow or brownish in aqueous solution due to charge transfer transitions.

Reaction 1: P to Q upon addition of HNO₃

Complex P reacts with aqueous HNO₃ (an acid) to form a very pale violet-colored complex Q. Let's consider the proposed structure of P from option 1, \( \text{[Fe(H}_2\text{O)}_5\text{(OH)]}^{2+} \). This complex contains a hydroxide ligand \( \text{(OH)}^- \). Adding an acid \( \text{(H}^+ \text{ from HNO}_3\text{)} \) will protonate the hydroxide ligand, turning it into a water molecule \( \text{(H}_2\text{O)} \).

The reaction would be:

The proposed complex Q in option 1 is \( \text{[Fe(H}_2\text{O)}_6\text{]}^{3+} \). This reaction transforms the \( \text{[Fe(H}_2\text{O)}_5\text{(OH)]}^{2+} \) complex into the hexaaquairon(III) ion, \( \text{[Fe(H}_2\text{O)}_6\text{]}^{3+} \). The hexaaquairon(III) ion is typically pale violet or almost colorless in dilute solutions. This matches the observed color change from yellow (P) to pale violet (Q).

Reaction 2: Q to R upon reaction with NaCl

Complex Q reacts with NaCl to form a yellow-colored complex R. NaCl provides chloride ions \( \text{(Cl)}^- \). Complex Q is \( \text{[Fe(H}_2\text{O)}_6\text{]}^{3+} \). Chloride ions are known to be ligands that can displace water molecules in coordination complexes.

The reaction would be a ligand substitution:

The proposed complex R in option 1 is \( \text{[Fe(H}_2\text{O)}_5\text{(Cl)]}^{2+} \). Chloro complexes of iron(III), like \( \text{[Fe(H}_2\text{O)}_5\text{(Cl)]}^{2+} \) or even \( \text{[FeCl}_4\text{]}^- \), are typically yellow. This matches the observed color change from pale violet (Q) back to yellow (R).

Summary of Species P, Q, and R

Based on the reactions and color changes described in the question, the species P, Q, and R consistent with the observations are:

• P: \( \text{[Fe(H}_2\text{O)}_5\text{(OH)]}^{2+} \) (yellow)
• Q: \( \text{[Fe(H}_2\text{O)}_6\text{]}^{3+} \) (pale violet)
• R: \( \text{[Fe(H}_2\text{O)}_5\text{(Cl)]}^{2+} \) (yellow)

These species and their transformations align perfectly with the reactions and colors given in the problem statement.