# Why there are more examples of carbonylate anions than metal carbonyl cations?

I know that metal carbonyl complexes that do not follow EAN rule or 18 electron rule try to achieve that by gaining electron (forming carbonylate) or loosing electron (forming carbonyl cations). But this question is way too dicey. It came in a national level exam in our country. I am yet to find a solution.

Yes there are more carbonylate ions than metal carbonyl cations, but the latter do exist. A salt $$\ce{[Mn(CO)6^+][BF4^-]•SO2}$$ has been synthesized in a superacid medium.
Going back to the referenced salt, we can see features that protect $$\ce{[Mn(CO)6^+]}$$ from such an ignominious fate. First note the counterion, $$\ce{[BF4^-]}$$which is a very weak nucleophile and has its negative charge buried inside the relatively bulky fluorine "shield". Then note the superacid medium; perforce the solvent is not competing as a Lewis base either.