Carbonyl/Nitrosonium Complex Stability
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. 

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