Before You Begin

Question #44574

Difficulty: 8/10 | 🟢 Free

The Question

Consider the dinuclear metal carbonyl \ce{Co2(CO)8}, which exists predominantly as a bridged isomer ( $C_{2v}$ symmetry) in the solid state at $298\text{ K}$ . To satisfy the 18-electron (EAN) rule, the complex possesses a Total Valence Electron Count (TVEC) of $w$ before metal-metal bonding, forcing the formation of $x$ \ce{Co-Co} bond(s). The solid-state structure contains $y$ bridging \ce{CO} ligands.

Upon reaction with one equivalent of \ce{PPh3}, a monosubstituted complex \ce{Co2(CO)7(PPh3)} is formed. Assuming the original bridged core framework is strictly maintained, let $z$ be the number of theoretically possible, structurally distinct geometric isomers (excluding enantiomers) for this monosubstituted complex.

A student incorrectly ignores the \ce{Co-Co} bond, assuming each Co atom is simply 5-coordinate with a regular Trigonal Bipyramidal (TBP) geometry where the bridging ligands occupy the axial positions.

Based on the correct application of the EAN rule and the resulting local idealized octahedral geometry around each Co atom, identify the correct values of $w$ , $x$ , $y$ , and $z$ .

Options

A.
$w = 34,\; x = 1,\; y = 2,\; z = 2$
B.
$w = 34,\; x = 1,\; y = 2,\; z = 1$
C.
$w = 36,\; x = 0,\; y = 2,\; z = 1$
D.
$w = 36,\; x = 1,\; y = 0,\; z = 2$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Geometric Isomerism Requirements|Synergic Bonding in Carbonyls|EAN Rule (Sidgwick's Rule)

Why This Question Is Tricky

Common Mistakes

This problem tests the student's ability to recognize that the 18-electron (EAN) rule dictates the formation of a Metal-Metal bond, which acts as a stereochemically active 'ligand' in the coordination sphere.

Students often fall into the trap of ignoring the M-M bond and applying standard VSEPR theory to the ligands alone (e.g., assuming a 5-coordinate Trigonal Bipyramidal geometry for the Co centers, which would make all terminal COs equivalent).

By accounting for the M-M bond, the effective coordination number becomes 6 (distorted octahedral), which breaks the equivalence of the terminal CO ligands and correctly predicts multiple geometric isomers upon substitution.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Geometric Isomerism Requirements
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free