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

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