How will u justify a lone pair on carbon in CO ?

By formal charge separation..........

A typical Lewis structure used for the carbon monoxide molecule is ##""^(-):C-=O^(+)##. Of course this is a formalism, that is it is a way to view the molecule in an abstract sense. However, such a Lewis structure does reflect the experimental fact that when carbon monoxide binds to a metal centre, it usually binds thru the carbon.