Question

which one is more Pi acceptor ligand out of CO and n2 and why

Answer 1

Its CO. First of all N2 is a pathetic sigma donor due its non polarity. N2 is infact reduced in its complexes. CO on the other hand has negative charge over carbon atom so it becomes a powerful sigma donor. After donation back bonding occurs between metal atom and anti bonding orbital of carbon. The positive charge over oxygen is also neutralised resulting in very high stability. So its a strong pie acceptor.

Thanks for asking. Hope this helps.