Write notes on synergic bonding.

Hint: Metal carbonyls are coordination complexes of transition metals with carbon monoxide ligands. Metal carbonyls are useful in organic synthesis and as catalysts or catalyst precursors in homogeneous catalysts.

Complete Solution :
- Synergic bonding is also known as Pi-backbonding. It is usually used in the context of organometallic chemistry where there is a transition metal centre and good pi-acceptor ligands like CO. In such a case, there are two bonding interactions at a time. Firstly donation of electrons from the ligand to an empty orbital on the which gives the typical ligand-metal interaction. However if the metal has filled d-orbitals, a second interaction can occur where the electrons are donated from the filled d orbitals on the metal to an empty orbital of the ligand (typically a Pi antibonding orbital). Together, these two interactions make up synergic bonding.
- Synergic bonding involves transference of electrons from ligands to metal and The transference of electrons from filled metal orbitals to anti-bonding orbitals of ligands. It's a bond between a carbonyl group acting as a ligand and a metal. Synergic bonding means self strengthening bond.
- In a metal carbonyl, the metal-carbon bond possesses both $\sigma $ and $\pi $ character. The bond between the carbonyl molecule and the metal is further strengthened by the synergic effect produced by the metal-ligand bond.

Note: Metal carbonyls are relatively distinctive in forming complexes with metals in negative oxidation states. Examples include the anions discussed above. These anions can be protonated to give the corresponding metal carbonyl hydrides. The neutral metal carbonyl hydrides are often volatile and can be quite acidic.