The number of bridging CO ligand(s) and Co—Co bond(s) in Co2(CO)8, respectively are
(a) 2 and 0
(b)

Q: Why does cobalt form a bond with another cobalt atom here?

Cobalt has 9 valence electrons. In $\text{Co}_2(\text{CO})_8$, the total valence electron count is 34. To satisfy the 18-electron rule for both cobalt atoms (which requires 36 electrons), they must share a pair of electrons, forming one $\text{Co—Co}$ bond.

Q: Are all the carbonyl ligands bridging?

No, only two of the eight carbonyl ligands act as bridges between the two cobalt atoms. The remaining six are terminal ligands, meaning they are attached to only one cobalt atom each.

Q: Does this structure remain the same when dissolved in a solvent?

Interestingly, no! In solution, the bridged structure exists in equilibrium with an unbridged isomer where all eight CO ligands are terminal and the only connection between the halves is the $\text{Co—Co}$ bond.

Q: How do chemists know there are bridging CO ligands?

Infrared (IR) spectroscopy is the key. Bridging CO ligands have a lower stretching frequency (around $1850 \text{ cm}^{-1}$) compared to terminal CO ligands (around $2000 \text{ cm}^{-1}$), allowing chemists to easily distinguish them.

Q: What is the oxidation state of cobalt in this complex?

The oxidation state of cobalt is zero. Carbon monoxide is a neutral ligand, and the complex overall is neutral, so the metal retains its elemental oxidation state.

Question

The number of bridging CO ligand(s) and $	ext{Co—Co}$ bond(s) in $	ext{Co}_2(	ext{CO})_8$, respectively are

Options:

(A) 2 and 0

(B) 0 and 2

(C) 4 and 0

(D) 2 and 1

Sigma Percentile · Accepted Answer

### Unveiling the Architecture of Dicobalt Octacarbonyl

Imagine you are an architect, but instead of bricks and mortar, you are building with atoms and electrons. Today, we are looking at a fascinating molecular structure: dicobalt octacarbonyl, or $	ext{Co}_2(	ext{CO})_8$. This is a classic example of a polynuclear metal carbonyl, and understanding its structure is a rite of passage for any chemistry student.

### The 18-Electron Rule: Predicting Metal-Metal Bonds

Before we even draw the molecule, we can predict its core structure using the 18-electron rule. Let's calculate the Total Valence Electrons (TVE) for the entire complex.

Cobalt is in Group 9 of the periodic table, so each cobalt atom brings 9 valence electrons. Carbon monoxide (CO) is a neutral ligand that donates 2 electrons.

$$ 	ext{TVE} = 2(9) + 8(2) = 18 + 16 = 34 	ext{ electrons} $$

For two metal atoms to independently satisfy the 18-electron rule, they would need a total of $2 	imes 18 = 36$ electrons. However, our complex only has 34 electrons. It is exactly 2 electrons short!

To make up for this deficit, the two cobalt atoms must share a pair of electrons, forming a direct metal-metal bond.

$$ 	ext{Number of M-M bonds} = \frac{36 - 34}{2} = 1 $$

So, we have mathematically proven that there is exactly **one $	ext{Co—Co}$ bond**.

### Bridging vs. Terminal Ligands

Now, how do the eight CO ligands arrange themselves around this $	ext{Co—Co}$ core? Nature loves symmetry and stability. In the solid state, two of the eight carbon monoxide ligands decide to act as bridges. They connect both cobalt atoms simultaneously, forming a rigid, butterfly-like central core. These are our **two bridging CO ligands**.

This leaves six CO ligands. Like loyal guards, they distribute themselves equally. Three of them attach exclusively to the left cobalt atom, and the other three attach exclusively to the right cobalt atom. These are known as terminal ligands.

So, counting them up, we have **2 bridging CO ligands** and **1 $	ext{Co—Co}$ bond**.

### The Solid vs. Solution State Mystery

Here is a pro-tip that often appears as a trap in competitive exams. The beautiful bridged structure we just discussed is the stable form in the **solid state**.

However, if you dissolve $	ext{Co}_2(	ext{CO})_8$ in a solvent, it exists in a dynamic equilibrium with an unbridged isomer. In this unbridged form, all eight CO ligands are terminal (four on each cobalt), and the only connection between the two halves is the single $	ext{Co—Co}$ bond. Always read the question carefully to see if it specifies the state, but by default, we assume the standard solid-state bridged structure!

Animated Solution for Chemistry - Coordination Compounds: The number of bridging CO ligand(s) and $Co—Co$ bond(s) in $Co_{2} (CO)_{8}$ , respectively are

Select Answer:

Visualized Solution

\text{Molecule Introduction}

\text{Metal-Metal Bond}

\text{Bridging Ligands}

\text{Terminal Ligands}

\text{Final Count}

\text{Equilibrium Fact}

The Sigma Insight: Bonding and Crystal field

Unveiling the Architecture of Dicobalt Octacarbonyl

The 18-Electron Rule

Bridging vs

The Solid vs

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