Hi

 

Carbonyl Complexes are compounds that contain carbon monoxide as a coordinated ligand. Carbon monoxide is a common ligand in transition metal chemistry, in part due to the synergistic nature of its bonding to transition metals. We can describe the bonding of CO to a metal as consisting of two components. The first component is a two electron donation of the lone pair on carbon (coordination exclusively through the oxygen is extremely rare) into a vacant metal d-orbital. This electron donation makes the metal more electron rich, and in order to compensate for this increased electron density, a filled metal d-orbital may interact with the empty pi* orbital on the carbonyl ligand to relieve itself of the added electron density. This second component is called pi-backbonding or pi-backdonation.

 

The two components of this bonding are synergistic. The more sigma donation by the carbonyl (or other sigma-donors on the metal center), the stronger the pi-backbonding interaction.

 

This occupation of the pi* on CO does lead to a decreased bond order in the carbon monoxide molecule itself. As we might expect, as the pi-backdonation becomes stronger, the CO bond order should decrease from that of the free ligand. Two consequences that we might expect if the CO bond order was reduced would be a lengthening of the C-O bond and a decrease in the carbonyl stretching frequency in the IR. Both of these hold true

 

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