Understanding the Bond Order of CO: Why It's 3.5 and Not 2.5

When discussing molecular structure and chemical bonding, one key concept is the bond order. For diatomic molecules and ions, the bond order helps us understand the strength and nature of the chemical bond. The CO- ion (carbon monoxide ion) is one such case where the bond order plays a significant role.

Bond Order Calculation

The bond order is calculated using the formula:

[text{Bond Order} frac{N_b - N_a}{2}]

Where:

N_b number of bonding electrons N_a number of antibonding electrons

Molecular Orbital Configuration of CO

Let's dissect the molecular orbital (MO) configuration of CO to understand the bond order of the CO- ion better.

Neutral CO Molecule

The electronic configuration of the neutral CO molecule is as follows:

Total electrons 10 (6 from Carbon and 8 from Oxygen) Molecular orbitals fill as follows: (sigma1s^2) (sigma^1s^2) (sigma2s^2) (sigma^2s^2) (sigma2p_z^2) (pi2p_x^2) (pi2p_y^2) (pi^2p_x^0) (pi^2p_y^0)

This results in:

Bonding electrons 8 (from (sigma2p_z), (pi2p_x), and (pi2p_y)) Antibonding electrons 2 (from (sigma^1s) and (sigma^2s))

Thus, the bond order for neutral CO is:

[text{Bond Order}_{CO} frac{8 - 2}{2} 3]

CO- Ion

To form CO- from the neutral CO molecule, one electron is removed from the highest energy molecular orbital, which is the (pi^2p_x) or (pi^2p_y) (both are degenerate). Removing one electron results in:

Bonding electrons 8 (unchanged) Antibonding electrons 1 (after removing one electron from the two (pi) orbitals)

Thus, the bond order for CO- is:

[text{Bond Order}_{CO^} frac{8 - 1}{2} 3.5]

Summary

The bond order of CO- is 3.5 because removing one electron from the neutral CO molecule decreases the number of antibonding electrons while the number of bonding electrons remains the same, leading to a higher bond order. A bond order of 2.5 would not accurately represent the correct counting of bonding and antibonding electrons based on molecular orbital theory for CO-.

Measured CO Bond Length and Bond Order

The measured bond length of CO is 1.128 ?, while the bond length of CO (neutral) is 1.115 ?. The bond order for CO is 3.5. The highest occupied molecular orbital (HOMO) for CO- is the (sigma 2s) MO. The bond order is defined as the number of electrons in bonding MOs minus the number of electrons in antibonding MOs.

It's important to note that the order of energy of the orbitals is not the same as it would be for homonuclear diatomic molecules like O2 where both atoms are the same. For heteroatomic diatomic molecules, the energy of bonding molecular orbitals (BMOs) closely matches those of the more electronegative atom, and the energy of anti-bonding molecular orbitals (ABMOs) closely matches those of the less electronegative atom.