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Question #44862

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The Question

Consider the isoelectronic series of octahedral metal carbonyls: $\ce{[Ti(CO)6]^{2-}}$ , $\ce{[V(CO)6]^{-}}$ , and $\ce{Cr(CO)6}$ . The standard aqueous redox potentials $E^\circ(\ce{M^{3+}/M^{2+}})$ for these metals are highly variable, with Ti and V exhibiting highly negative values, indicating a strong thermodynamic preference for higher oxidation states in water. Despite this, their low-valent carbonyl complexes are stable and exhibit intense colors (e.g., $\ce{[V(CO)6]^{-}}$ is intensely yellow), unlike their pale $\ce{[M(H2O)6]^{2+}}$ counterparts. Which of the following statements correctly identifies the primary origin of this intense color and the correct trend in the C-O stretching frequency ( $\nu_{\ce{CO}}$ )?

Options

A.
Color is due to MLCT; $\nu_{\ce{CO}}$ trend is $\ce{[Ti(CO)6]^{2-}} < \ce{[V(CO)6]^{-}} < \ce{Cr(CO)6}$ ; aqueous $E^\circ$ is thermodynamically decoupled from these transitions.
B.
Color is due to Laporte-allowed d-d transitions; $\nu_{\ce{CO}}$ trend is $\ce{[Ti(CO)6]^{2-}} > \ce{[V(CO)6]^{-}} > \ce{Cr(CO)6}$ ; negative $E^\circ$ lowers $\Delta_o$ .
C.
Color is due to LMCT; $\nu_{\ce{CO}}$ trend is $\ce{[Ti(CO)6]^{2-}} < \ce{[V(CO)6]^{-}} < \ce{Cr(CO)6}$ ; negative $E^\circ$ drives ligand-to-metal electron transfer.
D.
Color is due to MLCT; $\nu_{\ce{CO}}$ trend is $\ce{Cr(CO)6} < \ce{[V(CO)6]^{-}} < \ce{[Ti(CO)6]^{2-}}$ ; MLCT energy is directly proportional to $E^\circ$ .

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Explanation

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Concepts Behind This Question

E° (M³⁺/M²⁺) Redox Potential|Synergic Bonding in Carbonyls|Color and Charge Transfer

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to decouple thermodynamic aqueous redox potentials ( $E^\circ$ ) from the spectroscopic properties of organometallic complexes.

The 'Lethal Collision' occurs when students falsely correlate a highly negative $E^\circ$ (a strong reducing agent in water) with either Ligand-to-Metal Charge Transfer (LMCT) or altered d-d transition rules.

Furthermore, it tests the precise application of synergic bonding principles: recognizing that increased negative charge on the metal increases $\pi$ -back bonding, which populates the $\pi^*$ orbital of CO, thereby weakening the C-O bond and lowering its stretching frequency.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	D and F Block Elements - Class 12
Concept	E° (M³⁺/M²⁺) Redox Potential
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free