[thordike]

Can someone tell me why HCo(CO)₅ is a much stronger acid than H₂Fe(CO)₄?

I think it could be the change of coordination?
5 to 4 is better than 6 to 5?

Thank you very much

[Demotivator]

It's a stronger acid because the negative charge on Co(CO)5 ]-   is stabilized to a greater extent, because there are more carbonyl ligands,  than the tetracarbonyl.  Carbonyls have the ability to delocalize the charge through pi back bonding.

[thordike]

I'm very sorry, made a mistake!
It is H(CO)₄!
As far as I know H(CO)₅ does not exist.

I thought myself in the same direktion, but could not explain, why (with the correct stoichiometry) the effect on Co should be stronger than on Fe, except of the coordination sphere.

Still looking for an answer.
Sorry for the wrong formula!

[Demotivator]

Going from left to right with transition metals (True for elements in general) results in more protic  (and less hydridic) character for the hydrides. Probably due to stabilizing effect of increasing effective nuclear charge.