In which of the following transition metal ion (aquated) is not correctly matched with its colour?
Show Hint
Memorize the colours of common aqueous transition metal ions:
- Fe\(^{2+}\): Pale Green
- Fe\(^{3+}\): Yellow/Brown
- Cu\(^{2+}\): Blue
- Mn\(^{2+}\): Pale Pink
- Co\(^{2+}\): Pink
- Ni\(^{2+}\): Green
- Cr\(^{3+}\): Green/Violet
The colour of aqueous transition metal ions is due to d-d electronic transitions, where an electron absorbs light of a specific energy (and colour) to jump from a lower energy d-orbital to a higher energy d-orbital. The observed colour is the complementary colour of the light absorbed.
Let's examine the options:
(A) Fe\(^{2+}\): The aqueous ion is [Fe(H\(_2\)O)\(_6\)]\(^{2+}\). It absorbs light in the red part of the spectrum, and the transmitted light appears pale green. This match is correct.
(B) Cu\(^{2+}\): The aqueous ion is [Cu(H\(_2\)O)\(_6\)]\(^{2+}\). It absorbs orange-red light, and the solution appears light blue. This match is correct.
(C) Fe\(^{3+}\): The aqueous ion is [Fe(H\(_2\)O)\(_6\)]\(^{3+}\). Due to hydrolysis, solutions of Fe\(^{3+}\) are often yellow or brown. However, in dilute, acidic solutions to prevent hydrolysis, the hexaaqua iron(III) ion is very pale violet or pale pink. The characteristic colour strongly associated with Fe\(^{3+}\) in qualitative analysis (e.g., with thiocyanate) is blood-red, and in general aqueous solution, it's yellow/brown. Pink is generally associated with Mn\(^{2+}\). A yellow/brown colour would be a better description. Therefore, the match Fe\(^{3+}\) - Pink is questionable and likely incorrect in this context.
(D) V\(^{3+}\): The aqueous ion is [V(H\(_2\)O)\(_6\)]\(^{3+}\). It is known to be green. This match is correct. (V\(^{2+}\) is violet, VO\(^{2+}\) is blue, VO\(_2^+\) is yellow).
Comparing the options, the least accurate and most likely intended incorrect match is Fe\(^{3+}\) - Pink. The typical colour observed for Fe\(^{3+}\)(aq) is yellow or brown.
