Which of the following metal carbonyls has a M–M bond bridged by two CO groups?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → [Co2(CO)8]
In metal carbonyls, CO binds either as a terminal ligand (attached to one metal atom) or as a bridging ligand (shared by two metal atoms). In [Co2(CO)8] two of the CO groups are bridging ligands across the Co–Co bond.
Why the other options are incorrect
Option (1) [Cr(CO)6] — Incorrect. It has only one metal atom, so there is no M–M bond; it is octahedral.
Option (2) [Mn2(CO)10] — Incorrect. It has an Mn–Mn bond, but the bond is not bridged; all CO groups are terminal.
Option (4) [Fe(CO)5] — Incorrect. It has only one metal atom; it is trigonal bipyramidal.
NCERT (2026-27 edition):
Bonding in Metal Carbonyls
