[kelvinLTR]

What is the Oxidation state of Co in the complex (CO)₃Co(μ-CO)₂Co(CO)₃ ?

My argument was when CO moves to the bridging position, the Triple bond between C and O donates one electron pair to O, which makes C in CO a six electron species. Then the lone pair on C is split into two empty orbitals and they form σ bonds with Co metal.

And this argument gives me an oxidation Number of +2 for the metal.

But in the lecture, the lecturer told "Since CO is a neutral ligand, no matter what position it is in, in will not change the oxidation number." And thus said the Oxidation state of Co is 0.

Can someone please clarify this. And it will be great if you can attach a reference material so that I can speak with the lecturer with some reference.

PS: There is a Co-Co bond in the complex, thus it obeys the 18e rule.

[kriggy]

1) yes CO ligands are neutral so Co is in 0 oxidation state.
2) I dont think there is Co-Co bond (in this isomer), the compound is used as catalyst so I think 18 electrons would make that impossible
structure determination by X-ray says no Co-Co bond there
http://journals.iucr.org/q/issues/1964/06/00/a04222/a04222.pdf

[kelvinLTR]

The journal requires a password to be viewed.

But then, what is wrong with my argument?

[kriggy]

Well because when you form coordination  bond between ligand and metal you need electron pair to form it.
The more likely thing is, that the triple bond donates 2 electrons to the carbon which will formaly have negative charge and oxygen with positive and then you have 2 electron pairs which can be donated to two atoms of cobalt.
I dont have any reference for you im just browsing my notes from lectures, but sadly they are pretty brief.

[kelvinLTR]

I appreciate it 

But if Oxygen donates another pair of electrons, it would mean +2 charge on Oxygen and an octet unfilled.

[kriggy]

Im not realy sure how is the bonding in CO when its bridging ligand. This topic was covered briefly so I think its better to ask your profesor for explanation. He surely knows about this topic much more than me