[Winga]

When NO ligand binds to a metal centre, if NO donates an e- to M from its pi* orbital, it will become NO+. This NO+ is a good pi-acceptor ligand, and there is back-pi-bonding from M.

My question is that the NO ligand uses its pi*orbitals for both donating and receiving e-, so, overall, it is donating or receiving?
If there is back-pi-bonding to NO+ pi* orbitals, is it still NO+?

[AgG]

an easy way to tell would be to find data for the IR stretches of free NO and coordinated NO.  if the IR stretch of the coordinated NO is lower than the IR stretch of the free NO then it is a better receiver of electron density.  (remember that the lone electron is in a pi* - antibonding orbital.) if the IR stretch is higher, well, you get the idea.