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Question #44555

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The Question

Consider the synergic bonding in the following metal carbonyl complexes and their respective C-O bond lengths ( $d_{C-O}$ ): (I) Terminal CO in \ce{[Mn(CO)6]+} ( $d_1$ ) (II) Terminal CO in \ce{[Ni(CO)4]} ( $d_2$ ) (III) $\mu_2$ -bridging CO in \ce{[Fe2(CO)9]} ( $d_3$ ) (IV) Terminal CO in \ce{[Fe(CO)4]^{2-}} ( $d_4$ )

Which of the following options correctly identifies the increasing order of $d_{C-O}$ and provides the accurate primary chemical reasoning for the species with the maximum bond length?

Options

A.
$d_1 < d_2 < d_3 < d_4$ ; Maximum $d_{C-O}$ in (IV) is due to the highly negative formal charge on Fe (-2) maximizing $d\pi \rightarrow \pi^*$ back-donation, overcoming the typical dominance of $\mu_2$ -bridging denticity.
B.
$d_1 < d_2 < d_4 < d_3$ ; Maximum $d_{C-O}$ in (III) is due to the $\mu_2$ -bridging CO accepting electron density from two metal centers simultaneously, which always exceeds back-donation into a terminal CO.
C.
$d_2 < d_1 < d_3 < d_4$ ; Maximum $d_{C-O}$ in (IV) is due to the $sp^3$ hybridization of Fe, which provides better directional overlap for synergic bonding compared to the $d^2sp^3$ hybridization in (I).
D.
$d_4 < d_3 < d_2 < d_1$ ; Maximum $d_{C-O}$ in (I) is due to the positive charge on Mn contracting its d-orbitals, forcing the CO ligand to expand its bond length to maintain orbital overlap.

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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

Ligand Denticity Variations|Synergic Bonding in Carbonyls|Variable Oxidation State Stability

Why This Question Is Tricky

Common Mistakes

This question tests the boundary between 'Ligand Denticity Variations' and 'Oxidation State Fluctuation'.

Students often memorize the heuristic that bridging carbonyls ( $\mu_2$ , $\mu_3$ ) always exhibit greater pi-back bonding (and thus longer C-O bonds) than terminal carbonyls.

The trap is Option B.

The 'Hidden Variable' is the extreme oxidation state of the metal.

The -2 formal charge on Fe in Collman's reagent (\ce{[Fe(CO)4]^{2-}}) creates an exceptionally electron-rich $d^{10}$ center.

The magnitude of back-donation from this highly reduced mononuclear center into a terminal CO eclipses even the combined dual-metal back-donation into a $\mu_2$ -bridging CO in a neutral cluster like \ce{[Fe2(CO)9]}.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Ligand Denticity Variations
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free