Bonding in a bridged metal carbonyl Can someone please explain the bonding in a bridged metal carbonyl (like diiron nonacarbonyl)?

I cannot understand what kind of bonds (sigma/pi) exist between the bridging carbonyls and the metals.

Also why aren't the metals charged in such a case?
What is the bond order in such a case?
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