Bonding in a bridged metal carbonyl
I don't like to reinvent wheel -- if you go to Wikipedia's  Metal carbonyl and follow just two 1st literature position, you'll find your answer concerning bridging.

For MO calculations and a general scheme applicable to systems featuring “dative” MM bonds and non-compensating, semi-bridging carbonyl ligands, see: R. D. Barr, T. B. Marder, A. G.
Orpen, I. D.Williams, J. Chem. Soc. Chem. Commun. 1984, 112–114