Explain syergic interaction in metal carbonyls
Answers
Synergistic bonding is the more formal name for back bonding.
A carbonyl group donates it’s pair of electrons to an empty metal d orbital. This is a sigma bond and involves the lone pair in the valence sigma orbital of the carbonyl and the dx2-y2 or dz2 (or sometimes p) metal orbitals. The carbonyl is acting like a Lewis base.
When this happens there is very good overlap between the metals other d orbitals (dxy, dxz, dyz) with an empty pi* (antibonding) orbital on the carboyl group. The overlap allows the metal to donate some of it’s excess electron density into the aforementioned carbonyl orbital. This forms a pi bond. The carbonyl is acting like a Lewis acid.
This synergistic effect strengthens the metal carbonyl interaction. It also weakens the CO triple bond (as electron density is being inserted into a antibonding orbital on CO). Therefore TM carbonyls display weaker CO stretches in IR spectroscopy.
Here’s something to think about. TM carbonyl IR stretches are lower than free carbon monoxide (2143cm-1). In the carbon monoxide borane adduct OC-BH3 the vibrational band is higher than free carbon monoxide (2165 cm-1). How can you rationalise this?