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Hi, I have a doubt about the ligand CO in  organometallic compounds
Please hel me,is important!!!

PREMISE:
I know that the lewis structure of CO is:
|C≡O|   <---->  |C=O||


Now if I have an organometallic compound like M(CO)₄ :

---CO
-----|
CO-M-OC
-----|
---CO
I can say that each CO ligand "give" a lone pair (the lone pair of "C") to the metallic centre... a  "kind" of dative bond


MY DOUBT:  is when I have a BRIDGING  CO ligand :   a CO "linked"to TWO metallic "atoms" , Me.

-------CO
------/---\
----Me---Me

I don't understand the electronic situation of the CO in this compound....

a Usually (in this type of "bridge compounds" )are the two bond with  (2) Me two dative bond ??
How can CO donate two lone pair?? ....C has only 1 lone pair.

OR

b)  is a three-center two-electron bond??  Where C in CO donates its lone pair at the two metallic centers??

Thanks !!