Is CO ligand an important ligand tomake stable complex?

Carbonyls are indeed strong field ligands — they undergo pi backbonding (electrons from the HOMO of the metal atom donate into the pi* LUMO of the carbonyl as they have the same symmetry). Relative to sigma-only type ligands, pi ligands have greater overlap with the metal d-orbitals and increase crystal field splitting (ΔCFT).

All of this sounds fine theoretically, but how can we confirm? Take a look at the bond order of bound and unbound carbon monoxide. Carbon monoxide has a bond order of 3 (predicted from MO diagram and verified spectroscopically by an IR stretching wavenumber for the C-O bond of ~2200) while the carbonyl ligand has a bond order of 2 (predicted by donation of 2 electrons into the pi* orbitals and verified spectroscopically by a stretching wavenumber of ~1700)