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Question #44603

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The Question

The C-O bond in free carbon monoxide gas ( $\ce{CO_{(g)}}$ ) at $298\text{ K}$ exhibits a stretching force constant of $18.6\text{ mdyn/\AA}$ . Consider the isoelectronic series of octahedral metal carbonyl complexes: $\ce{[V(CO)_6]^-}$ , $\ce{[Cr(CO)_6]}$ , and $\ce{[Mn(CO)_6]^+}$ , analyzed via IR spectroscopy in $\ce{CH_2Cl_2}$ solution at $298\text{ K}$ . The synergic bonding mechanism ( $M(d\pi) \rightarrow CO(\pi^*)$ ) significantly alters the C-O bond strength. Which of the following sets correctly represents the C-O stretching force constants for $\ce{[V(CO)_6]^-}$ , $\ce{[Cr(CO)_6]}$ , and $\ce{[Mn(CO)_6]^+}$ , respectively?

Options

A.
$14.0, 16.3, 17.3 \text{ mdyn/\AA}$
B.
$17.3, 16.3, 14.0 \text{ mdyn/\AA}$
C.
$19.2, 18.6, 17.3 \text{ mdyn/\AA}$
D.
$14.0, 18.6, 20.1 \text{ mdyn/\AA}$

Check Your Answer

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

Synergic Bonding in Carbonyls|Oxidation State Traps

Why This Question Is Tricky

Common Mistakes

The question tests the 'Oxidation State Trap' within the context of Synergic Bonding.

Students often erroneously assume that a higher positive charge on the central metal strengthens all metal-ligand interactions uniformly.

However, for $\pi$ -acid ligands like $\ce{CO}$ , a higher positive oxidation state increases the effective nuclear charge ( $Z_{eff}$ ), which contracts the metal $d$ -orbitals.

This 'hidden variable' drastically reduces the metal's ability to back-donate electron density into the $\ce{CO}$ $\pi^*$ antibonding orbital.

Consequently, textbook learners miss this electronic competition and fail to predict the 'anomalous' strengthening of the C-O bond (higher force constant) in positively charged complexes.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Synergic Bonding in Carbonyls
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free