To determine the HOMO (Highest Occupied Molecular Orbital) of CO, we need to consider its molecular orbital (MO) diagram.

Carbon monoxide (CO) has a total of 14 electrons (C: 6, O: 8). The MO configuration for diatomic molecules like CO (which is similar to N₂) is:

σ2s², σ*2s², π2p⁴, σ2p⁲

The order of energy levels for CO is generally considered as:

σ2s < σ*2s < π2p < σ2p < π*2p < σ*2p

Filling 14 electrons:

1. σ2s² (2 electrons)
2. σ*2s² (4 electrons)
3. π2p⁴ (8 electrons)
4. σ2p⁲ (10 electrons)

Wait, this is incorrect. The actual MO diagram for CO is more complex due to the difference in electronegativity between C and O, leading to significant mixing. However, for general purposes, the MO configuration for CO (14 electrons) is often considered similar to N₂, but with some modifications. The correct filling order for CO is:

σ₂₂², σ*₂₂², π₂p⁴, σ₂p⁲

The highest occupied molecular orbital (HOMO) in this configuration is the σ₂p orbital. This σ₂p orbital is primarily formed from the 2p₂ atomic orbitals of C and O. However, due to the difference in electronegativity, the σ₂p orbital in CO is largely localized on the carbon atom and has significant non-bonding character, especially from the carbon's lone pair. It is often described as a non-bonding molecular orbital (N.B.M.O) with a slight bonding character, or more accurately, a weakly bonding σ orbital that is primarily localized on carbon and acts as a lone pair.

In many contexts, especially when discussing the donor properties of CO, the HOMO is considered to be a non-bonding orbital (N.B.M.O) primarily localized on the carbon atom, which is responsible for its ligand behavior.

Option Analysis:

• A) ${\sigma }^{\ast }2{\mathrm{p}}_{\mathrm{z}}$: This is an antibonding orbital and would be the LUMO (Lowest Unoccupied Molecular Orbital) or higher, not the HOMO.
• B) N.B.M.O: The HOMO of CO is often described as having significant non-bonding character, primarily localized on the carbon atom, and is responsible for its donor properties. This is the most appropriate description among the given options.
• C) ${\pi }^{\ast }2\mathrm{py}={\pi }^{\ast }2{\mathrm{p}}_{\mathrm{z}}$: These are antibonding $\pi$ orbitals and are unoccupied, thus they are not the HOMO.
• D) None of these: Option B is a valid description.

Correct Answer: (B)