Bonding In Metal Carbonyls

What is the role of synergic bonding in metal-carbonyl complexes?

The metal-carbon bond in metal carbonyls prossesses :

Assertion : C - O bond in metal carbonyl is long. Reason : There is delocalisation of electrons from filled d orbitals into the empty orbitals on the CO ligands.

Among the following , metal carbonyls, the C-O bond is strongest :

Among the following , metal carbonyls, the C-O bond is strongest :

b) How is a metal-carbon pi bond formed in metal carbonyls?