[orgoclear]

What is the bond order of CO+ ?

This is what I did. I wrote the MO structure of CO and then removed the last electron. This gave me a bond order of 2.5 (CO had 3) but incidentally, the answer was given that the bond order is 3.5

Can anyone explain?

[cth]

What is the bond order of CO+ ?

This is what I did. I wrote the MO structure of CO and then removed the last electron. This gave me a bond order of 2.5 (CO had 3) but incidentally, the answer was given that the bond order is 3.5

Can anyone explain?

Good, that's what you should do to solve this problem.
And I found the same answer as you did, a bond order of 2.5. 

You can have a look at http://www.uky.edu/~holler/orbitals/co/co.html for a nice view of CO molecular orbitals. They start from the most stable one (1s of oxygen) to the less stable (σ*_2pz). In green, you can have a view at their spatial geometry.

The HOMO (highest occupied molecular orbital) is σ_2pz, which is bonding. If you extract an electron from there, the bond order decreases.

To have a bond order of 3.5, you would need the σ*_2s to be the HOMO. But it is not the case.

There seems to be a mistake with the answer of 3.5 you were given.

[orgoclear]

http://wiki.answers.com/Q/What_is_the_bond_order_of_CO_plus

it says 3.5

Now I have been looking in my inorganic textbook (JD Lee). It says, CO+ is an exception that has not been satisfactorily explained by MO theory. It has an exceptional BO of 3.5

http://books.google.com/books?id=q77rPHP5fWMC&pg=PA260&lpg=PA260&dq=CO%2B+bond+order&source=bl&ots=qQhIpQvC9r&sig=LE3wCy7Zabnus4EVc8wXG4Eu39E&hl=en&ei=DfEVS538Es-IkAWdn9yTBw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CAoQ6AEwATgK#v=onepage&q=CO%2B%20bond%20order&f=false

The bond length of CO > CO+.. This indicates that BO of CO must be less than CO+

[renge ishyo]

The BO in CO+ is 3.5 because the order of the orbitals is not the same as it would be for a homonuclear diatomic molecule such as O₂ where both atoms are the same. In the case of CO, the 2s atomic orbital on Oxygen is much lower in energy than the 2s atomic orbital in carbon. The discrepancy in energies allows the 2p-pi and 2p-sigma molecular orbitals to sink lower than the 2s-sigma* orbital in the MO diagram. For this case the calculations would be:

For CO:

2s-sigma²2p-pi_x² 2p-pi_y² 2p-sigma_z² 2s-sigma*²

BO = (8-2)/2 = 3

For CO+:

2s-sigma²2p-pi_x² 2p-pi_y² 2p-sigma_z² 2s-sigma*¹

BO = (8-1)/2 = 3.5

[orgoclear]

thanks renge ishyo.

But how do I know when to use what structure?

[renge ishyo]

The real answer? You need group theory coupled with spectroscopic data to get an accurate MO diagram. The MO diagrams that you study in beginning chemistry are Disneyland. They let you practice and learn the same concepts that you can apply to real MO diagrams, but the diagrams you are studying are idealized (i.e.not real). They avoid the complications of the orbitals shifting positions which happens all the time. Using such diagrams, you would get a BO of 2.5 because the real order of the orbitals in CO had been neglected.

You should just be aware that the order of the orbitals need not follow that simple order of filling that it does. In fact, there are variations in the order of filling even amongst simple molecules. For instance, the MO diagrams for the valence levels of Li₂, B₂, C₂, and N₂ are filled so that the 2p-pi orbitals fill before the single 2p-sigma orbital. It is only for O₂ and F₂ that the 2p-sigma orbitals come before the 2p-pi orbitals when you are filling. Since in this case the inversion doesn't involve antibonding orbitals you can ignore the actual filling order of the orbitals and still get correct bond orders (however, notice how in the CO configuration I wrote I was careful to place the 2p-pi orbitals before the 2p-sigma orbitals following the pattern in the order of filling set by C₂).

Sadly, it will be quite some time before you can get into Group Theory. It is normally introduced much later in your education (Inorganic Chemistry was my first exposure), and if you are a biology major you will likely never have to experience it