Stability of Carbonyl Complexes.

Jun 2014
The question is:

Why is V(CO)6 stable but Pt(CO)4 and Zn(CO)4 are unstable?

As far as I can see V complex is 17e and Pt complex 18e which would imply the opposite stabilities. For the Zn complex I have some ideas. It has an s2d10 config and d orbitals contract quickly across TM period. Therefore I would say that it doesn't have large enough orbitals to extend and overlap with CO pi* orbitals required for pi backbonding? Hence, not stable, as pi backbonding is a requirement for carbonyl complex stability.

Am I on the right lines for Zn complex? And can anyone help me with the Pt and V complexes?

Thanks guys. :)