Carbonyl/Nitrosonium Complex Stability Why is the nickel complex exhibiting the strongest pi-bonding? 

From what I understand, charge density plays a role. Also we need some empty orbitals on the metal cation, since we're looking at Lewis adducts. 

Also I note that some have positive overall charges; could this play a role? Metal cation &lt;-> NO+ repulsion? So we might want a metal anion? 

Also would size play a role? Size goes hand in hand with charge density. And what about "backbonding"?
Also we need some empty orbitals on the metal cation, since we're looking at Lewis adducts. 


Ahem. Nope. $\pi$-bonding in $\ce{CO/NO+}$ complexes is from back-donation, from occupied metal d-orbitals onto unoccupied $\pi^*$ orbitals of the ligand. 

Looks like homework question, so no further details provided.