The bond order of carbon monoxide changes when it acts as a ligand in metal carbonyls due to back-bonding into anti-bonding orbitals.
the verdict
COMMON KNOWLEDGE
no citation needed for this one
refutedsupported
the weight of evidence
no sources on either side
The retrieved sources do not contain evidence addressing metal carbonyl back-bonding and carbon monoxide bond order, but the claim represents standard chemical consensus and textbook coordination chemistry.