Q.Discuss briefly the nature of bonding in metal carbonyls.

Step 1. In a metal carbonyl, the M-CO bond is described as SYNERGIC, meaning it has two mutually reinforcing components acting at once.

Step 2. The first component is a sigma (σ) bond: the carbon atom of the carbonyl ligand donates its lone pair of electrons into an empty d orbital on the metal, forming the M ← CO sigma bond and making the metal electron-rich.

Step 3. To relieve this build-up of electron density on the metal, a filled metal d orbital then donates electron density back into the empty π* (antibonding) orbital of the C-O bond -- this second component is called pi (π) back-bonding, M → CO.

Step 4. So electron density flows from ligand to metal through the sigma bond, and from metal back to ligand through the pi bond -- a two-way, mutually reinforcing (synergic) flow. …

Describe both halves of the synergic bond -- sigma donation from ligand to metal, and pi back-donation from metal to ligand -- and explai …

• Describing only the sigma-donation half of the bond and omitting the pi back-bonding component, which is the part that specifically explains why metal carbonyl bonds are unusually strong. …