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Question #44198

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The Question

The brown binuclear complex $\ce{[(NH3)5Co(\mu-O2)Co(NH3)5]^{4+}}$ undergoes a one-electron oxidation to form the green complex $\mathbf{X}$ . In $\mathbf{X}$ , the dioxygen moiety acts as a bridging ligand between the two cobalt centers. Assuming the nuclear spin of $\ce{^{59}Co}$ is $I = 7/2$ and $\ce{^{16}O}$ is $I = 0$ , which of the following correctly identifies the spin-only magnetic moment ( $\mu_{eff}$ ), the approximate $\ce{O-O}$ bond order (B.O.), and the number of expected EPR hyperfine lines for complex $\mathbf{X}$ at 298 K?

Options

A.
$\mu_{eff} = 1.73 \text{ BM}$ , $\text{B.O.} = 1.5$ , $15 \text{ lines}$
B.
$\mu_{eff} = 1.73 \text{ BM}$ , $\text{B.O.} = 1.5$ , $1 \text{ line}$
C.
$\mu_{eff} = 1.73 \text{ BM}$ , $\text{B.O.} = 1.0$ , $8 \text{ lines}$
D.
$\mu_{eff} = 1.73 \text{ BM}$ , $\text{B.O.} = 1.0$ , $15 \text{ lines}$

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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

Molecular Orbital Mixing|Crystal Field Splitting (Octahedral)

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of Molecular Orbital Theory (MOT) and Coordination Chemistry.

The primary trap is the assumption that a superoxo ligand ( $\ce{O2^{-}}$ ) in a coordination complex behaves identically to an isolated gas-phase superoxide ion.

Students often correctly identify the 1.5 bond order and 1.73 BM magnetic moment but incorrectly assume the unpaired electron remains localized on the oxygen atoms (which would yield 1 EPR line since $\ce{^{16}O}$ has $I=0$ ).

In reality, the ligand's $\pi^*$ orbitals strongly hybridize with the metal's $d$ -orbitals due to crystal field symmetry, delocalizing the electron across both Cobalt centers and resulting in 15 EPR hyperfine lines.

Distractors C and D trap students who incorrectly assume the oxidation occurs at the metal center (forming a $\ce{Co^{IV}}$ species) rather than the non-innocent dioxygen ligand.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Molecular Orbital Mixing
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free