Why do we see a lot of oxides and fluorides bonding with f-block metals? Do they bond with polyatomic ions?

In general, f-block elements are not nearly as well-studied as we'd like when it comes to their bonding with polyatomic ions.

I guess this counts---plutonium(III) hydroxide has a picture on wikipedia... with no in-text mention of it. I could randomly find an entry via google of ammonium plutonium carbonate...

I also think #"PuCO"# is not impossible (if you consider #"CO"# polyatomic). I see no reason why the #sigma#-type and #pi#-type #f# orbitals on plutonium can't interact with #"CO"#, in which #"CO"# donates electron density via a #sigma# interaction to plutonium's #f_(z^3)# atomic orbital, and plutonium donates back from the #pi#-type #f# (#f_(xz^2)# and #f_(yz^2)#, I think they're called?) orbitals to the #"CO"# #pi^"*"# in a "backbonding" stabilization. One would need actual numbers to justify that though...

So, I guess I would say yes, but I don't think it's well-studied. My research group is working with many f-block elements computationally, but they are still sticking with triatomics and other small molecules due to the massive computational cost in using high-levels of theory and complicated basis sets. We're getting there though!