Problem 24 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\(\left[\mathrm{Co}_{2}(\mathrm{CO})_{8}\right]\) displays: (a) one Co-Co bond, six terminal \(\mathrm{CO}\) and two bridging CO (b) one Co-Co bond, four terminal \(\mathrm{CO}\) and four bridging \(\mathrm{CO}\) (c) no Co - Co bond, six terminal \(\mathrm{CO}\) and two bridging \(\mathrm{CO}\) (d) no Co- Co bond, four terminal \(\mathrm{CO}\) and four bridging \(\mathrm{CO}\)

Step-by-Step Solution

Answer

The correct option is (a): one Co-Co bond, six terminal CO, and two bridging CO.

Step 1: Understanding the Molecular Structure

The compound given is \([\mathrm{Co}_{2}(\mathrm{CO})_{8}]\). This indicates there are two cobalt (Co) atoms and a total of eight carbon monoxide (CO) ligands.

Step 2: Assessing Co-Co Bond Presence

Examine the structure for Co-Co bond formation. In a typical \([\mathrm{Co}_{2}(\mathrm{CO})_{8}]\) compound, there is a Co-Co bond present, as cobalt atoms are directly bonded to each other to stabilize the overall structure.

Step 3: Identifying Terminal and Bridging CO Ligands

Analyze the bonding of the CO ligands. In \([\mathrm{Co}_{2}(\mathrm{CO})_{8}]\), there are typically six terminal CO ligands, attached directly to the cobalt atoms, and two CO ligands that are bridging, connecting the two cobalt atoms.

Step 4: Choosing the Correct Option

Based on the information gathered, which includes one Co-Co bond, six terminal CO, and two bridging CO, the correct structural description is option (a): "one Co-Co bond, six terminal \(\mathrm{CO}\) and two bridging CO."

Key Concepts

Metal Carbonyls

Metal carbonyls are compounds consisting of metal atoms bonded to carbon monoxide ligands. These compounds are a fascinating area of coordination chemistry because they showcase how metals interact with ligands through unique bonding patterns. In

• metal carbonyls, the central metal atom is typically in a low oxidation state, which aids in the formation of stable bonds with CO molecules.
• The bonding is characterized by a synergistic effect where electrons are donated from the filled lone pairs of the CO's carbon atom to the empty orbitals of the metal, a process known as sigma bonding.
• Additionally, back-bonding may occur where electrons from filled metal orbitals are donated to empty antibonding orbitals of CO, creating pi back-bonding.

This dual electron sharing strengthens the overall bond between the metal and the carbon monoxide ligands, making metal carbonyl complexes like often highly stable.

Co-Co Bonding

The Co-Co bond in a critical feature of the compound. This bond is covalent, indicating that there is a sharing of electrons between the two cobalt atoms.

• The presence of a Co-Co bond in a compound like primarily aids in the stability of the overall molecular structure.
• This bond is particularly important because it allows the cobalt atoms to maintain a favorable electron count and enhances the molecule's electron-sharing capabilities.
• Such a bond is characteristic of many metal-metal bonded compounds in coordination chemistry, contributing to unique physical and chemical properties.

Understanding Co-Co bonds helps in comprehending how metal clusters and other complex metallic structures can be formed and stabilized in chemistry.

Ligand Classification

Ligands are atoms, ions, or molecules that donate at least one pair of electrons to a central metal atom or ion, forming a coordination bond. In coordination chemistry, it is essential to understand ligand classification and variations. In

• "terminal" ligands, such as the six CO ligands in attach directly to a single metal atom. These ligands typically result in straightforward, single-point coordination with the metal.
• "Bridging" ligands, like the two CO ligands in form bonds with two or more metal centers, effectively connecting them. This type of bonding can provide added structural integrity and stability to the compound.
• Different ligand types can drastically alter a metal complex's reactivity, stability, and electronic properties.

Mastering the classification of ligands thus allows chemists to predict how metal complexes will behave and how they can be utilized in various chemical processes.