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What is synergic bonding ?

Carbon monoxide, CO, as a ligand binds itself to metal atoms through the carbon atom. It is a weak donor (a weak base). It forms a weak sigma-bond to the central atom. CO is also an acceptor ligand and from a π-bond to the metal. This characteristic property of back bonding stabilised the metal-ligand interaction. 
For a better understanding of the nature of bonding in metal carbonyls, we have to consider first the molecular orbital energy level diagram of carbon monoxide given in the figure. 3-sigma, the highest occupied molecular orbital in CO is essentially a lobe projecting away from the carbon atom. When CO acts as a ligand. This orbital serves as a weak donor to the metal atom, and forms a bond as represented in Figure. The lowest unoccupied molecular orbltals of CO are the π* (2π) orbitals. These play an important part in bonding as they can overlap metal d orbltals having π symmetry. The resultant Interaction leads to the delocalisation of electrons from, filled d orbltals on the metal into the empty orbitals on the CO ligands.This in fact, is back bonding from metal to CO. The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal (Figure).
The MO energy level diagram for CO The filled 3-sigma and the vacant 2π-oribttols are important for bonding in the formation of metal carbonyls

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