MO diagram: carbon monoxide template

Valence MO diagram of CO (isoelectronic with N₂): O orbitals lower than C, 3σ, 4σ, 1π, 5σ HOMO on carbon, 2π* LUMO, bond order 3.

What the template shows

• A heteronuclear diatomic: unequal atomic orbital energies give polarised molecular orbitals
• Carbon monoxide
• O 2s and 2p lie below C 2s and 2p, so bonding MOs are mostly O and antibonding MOs mostly C. MOs numbered with the 1σ, 2σ core
• Bond order = ½ (8 − 2) = 3
• diamagnetic (all electrons paired)
• CO has 10 valence electrons, like N₂; counted the same way the bond order is 3.
• The HOMO (5σ) is largely a carbon lone pair, so CO binds metals through carbon (σ donation).
• The empty 2π* (LUMO), larger on carbon, takes electron density back from filled metal d orbitals (π back-bonding), as in Ni(CO)₄.
• Photoelectron spectrum: ionisation from 5σ at 14.0 eV, 1π at 16.9 eV and 4σ at 19.7 eV (5σ above 1π).
• Energies are schematic: O 2s and 2p lie lower than C 2s and 2p because O is more electronegative.
• Every part of this figure is editable · sources on the template card

Drawn with 228 shapes – every one editable in MolDraw.

Sources

• Miessler, Fischer & Tarr, Inorganic Chemistry, 5th ed., Pearson 2014, ch. 5 Textbook
• NIST Chemistry WebBook: carbon monoxide Database