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According to my books the bond order of $\ce{CO+}$ is $3.5$. But shouldn't it be $2.5$? On googling this, I found the following answer that is on Stack Exchange but its only talks about the bond length.

I am unable to understand why it is $3.5$ as I am in class 11.

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For a long time it was taught in school and universities that the HOMO of carbon monoxide is anti-bonding. Without more context it was also often taught that the bond order in CO is three, since there are eight electrons in bonding orbitals and two in anti-bonding orbitals. $$\text{Bond order} = \frac12(\text{bonding} - \text{anti-bonding})$$ By assuming that the HOMO is anti-bonding (it is not!) and taking away one electron, the bond order has to increase to 3.5. This is wrong.

When we have a look at the MO diagram, a calculated version can be found here, we know that the HOMO, i.e. 3σ, is a bonding orbital, while the anti-bonding orbital is the 2σ. Upon ionisation, we would indeed remove one bonding electron and therefore the bond order has to decrease to 2.5 as you suggested.
However, it is not that easy. Strictly speaking the below MO scheme is, as well as MO theory itself, an approximation, and only one possible configuration. While we do not have to use resonance structures with MO theory, we have to consider other configurations (analogous to excited states). So naturally the bond order of CO is not strictly 3. And removing an electron does not mean we are removing it from only one orbital, rather than decreasing the electron density. Therefore we cannot accurately predict the bond order with these simple considerations.
Experimental observations and theoretical calculations suggest that the bond indeed becomes stronger when removing an electron. See the linked question and Philipp's answer within for more detail. (Don't look at the other answers, they are as wrong as they could be.)

In short: The bond order of $\ce{CO}$ is not exactly 3 and removing an electron will not increase the bond order to 3.5. In both cases, the observed bond order is probably closer to 2.5, while experiments suggest that the bond is stronger in $\ce{CO+}$.

MO of CO

An orbital with bonding character has no node perpendicular to the bond axis; an orbital with anti-bonding character has at least one node perpendicular to the bond axis (electron density is zero). Strictly speaking there are no non-bonding orbitals.

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    $\begingroup$ Shouldn’t the last sentence be ‘… in carbon monoxide’? Iirc, there are a few nonbonding orbitals (due to symmetry) e.g. in $\ce{HCl}$. $\endgroup$ – Jan Jan 24 '17 at 21:39
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    $\begingroup$ @Jan That's why I said strictly, one of the two categories will fit any orbital. What we usually classify as non-bonding orbitals are linear combinations that "don't change in energy". That's simply not possible due to an external field. In HCl the non-bonding orbitals don't have a node perpendicular to the bond axis (you caught that I forgot that), so they are classifiable as bonding. $\endgroup$ – Martin - マーチン Jan 25 '17 at 1:30
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This is a very good explaination I found here: http://www.answers.com/Q/What_is_the_bond_order_of_CO_plus

CO is not a homo-nuclear atom like C2, N2 or even O3, O3 (both these categories are different: with and without 2s-2p mixing). So there is a large discrepancy in the atomic energy levels of 2s, 2p e- of C and O.As a result, 2p(pi)x, 2p(pi)y and 2p(sigma)z have lower energy than 2s(sigma)*. So the e- lost is from 2s(sigma)*2 and not 2p(sigma)z. And hence the bond order increases from 3 to 3.5 and not decreases to 2.5 . (The typical school textbook formula doesn't work for species like CO,CO+ and even NO,NO+ in many situations)

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  • $\begingroup$ I would highly disagree that you consider answers.com a good source, atleast in comparison to ChemSE. And second of all, it's discouraged to copy answers word-for-word. $\endgroup$ – Pritt Balagopal Jul 12 '17 at 13:38
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Due to small size and positive charge CO structure faces 2s antibonding repulsion and when electron is ejected out it is removed from 2s antibonding so bond order become 3.5enter image description here

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    $\begingroup$ This diagram is incorrect, it depicts C and O atomic energy levels being the same. "2s antibonding repulsion"? I really don't know what you're talking about. $\endgroup$ – orthocresol Nov 9 '16 at 11:14
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It is $3.5$ due to synergic bond in $\ce{CO}$ which release a lot of energy hence which upgrade the energy of antibonding $2\sigma$, and when we change $\ce{CO}$ to $\ce{CO+}$ electrone is released from $2\sigma$ antibonding, resulting in bond order of $3.5$.

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protected by orthocresol Jul 12 '17 at 12:14

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