What is the Difference Between Terminal and Bridging Carbonyls?

The key difference between terminal and bridging carbonyls lies in the number of metal atoms bonded to the carbon atom in the carbonyl group.

• Terminal Carbonyls: A terminal carbonyl group is a simple structure that uses its lone electron pair at the carbon atom to bind with a single metal atom. Terminal carbonyl compounds are typically characterized by a ν(CO) stretching band in the infrared spectroscopy, which appears at around 2100-2000 cm⁻¹. Some common examples of terminal carbonyl compounds include carbamates, derivatives of phosgene, lactams, etc.
• Bridging Carbonyls: A bridging carbonyl group is a more complex structure that has two or more metal atoms bonded to the carbon atom. Bridging carbonyls often undergo exchange with terminal carbonyls. In the case of a bridging carbonyl, the ν(CO) stretching band appears in the range of 1720-1850 cm⁻¹. An example of a bridging carbonyl is Fe₂(CO)9.

In summary, terminal carbonyls have a single metal atom bonded to the carbon atom, while bridging carbonyls have two or more metal atoms bonded to the carbon atom. The distinction between these two types of carbonyls can be observed through infrared spectroscopy, where the ν(CO) stretching band appears at different frequencies depending on whether the carbonyl is terminal or bridging.

Comparative Table: Terminal vs Bridging Carbonyls

The key difference between terminal and bridging carbonyls lies in the number of metal atoms bonded to the carbon atom in the carbonyl group. Here is a table summarizing the differences between terminal and bridging carbonyls:

Terminal carbonyl groups have a lone electron pair at the carbon atom that is used to bind to a single metal atom, resulting in a simple structure. On the other hand, bridging carbonyl groups have a complex structure, with the carbon atom bonded to two or more metal atoms. When using infrared spectroscopy, terminal carbonyl groups exhibit a stretching band of 2000 – 2100 cm^-1, while bridging carbonyl groups have a stretching band of 1720 – 1850 cm^-1.