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Question #44140

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The Question

An octahedral complex $X$ of a 3d transition metal $M$ in aqueous solution at $298 \text{ K}$ has a spin-only magnetic moment of $3.87 \text{ B.M.}$ and exhibits a characteristic visible absorption band at $\lambda_1$ due to a spin-allowed d-d transition. When excess $\ce{KCN(aq)}$ is added to the solution of $X$ under aerobic conditions (bubbling $\ce{O_2}$ ), a rapid electron transfer occurs, yielding a highly stable, diamagnetic octahedral complex $Y$ . Complex $Y$ exhibits its lowest-energy spin-allowed d-d transition at $\lambda_2$ . Assuming no charge transfer bands obscure the relevant spectra, which of the following correctly identifies $M$ , the relationship between the absorption wavelengths, and the dominant physical cause?

Options

A.
$M = \ce{Co}$ ; $\lambda_2 < \lambda_1$ ; Oxidation to $\ce{Co^{3+}}$ increases $Z_{\text{eff}}$ , which, combined with $\pi$ -acceptor $\ce{CN^-}$ ligands, drastically increases $\Delta_o$ .
B.
$M = \ce{Co}$ ; $\lambda_2 > \lambda_1$ ; The transition to a low-spin $d^6$ state pairs electrons in the $t_{2g}$ orbitals, increasing interelectronic repulsion and reducing $\Delta_o$ .
C.
$M = \ce{V}$ ; $\lambda_2 < \lambda_1$ ; Oxidation to $\ce{V^{5+}}$ yields a diamagnetic complex where the increased $Z_{\text{eff}}$ shifts the remaining d-d transitions into the ultraviolet region.
D.
$M = \ce{Cr}$ ; $\lambda_2 > \lambda_1$ ; Formation of a low-spin $d^4$ complex decreases the metal-ligand bond length, causing a relaxation in the crystal field splitting and a red-shift.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Electron Transfer Accounting|Complex Ion Oxidation States|Color and d-d Transitions

Why This Question Is Tricky

Common Mistakes

This problem tests the intersection of Crystal Field Theory, redox chemistry, and spectroscopy.

The primary trap is the 'Hidden Variable' of d-d transition prerequisites: students often correctly identify that V^5+ or Cr^6+ are diamagnetic, but fail to realize that a d^0 system cannot undergo d-d transitions, making Co the only chemically valid answer.

Furthermore, it tests the synergistic effect of increasing both the metal's oxidation state (Z_eff) and the ligand's spectrochemical strength on the magnitude of Delta_o, and inversely, the absorption wavelength.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Redox Reactions - Class 11 Part 2
Concept	Electron Transfer Accounting
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free