Carbonyl ligands and the eighteen electron rule

[pkgem]

Hi all,

 

I have a question on an assignment that requires me to draw a structure for a transition-metal carbonyl complex [Fe2(CO8]2- (I'm new to this forum and cannot see where to apply super/subscript).

To work out the structure, I first need to apply the eighteen electron rule. I have worked out that Fe is in the oxidation state +1 and thus each Fe ion contributes 7 electrons. There are 8 carbonyl ligands supplying a total of 16 electrons, that's eight each to each Fe ion - a total of 15 electrons. I know that the metal can be bonded to itself in these complexes but it there is an Fe-Fe bond then that would only supply one additional electron, taking my total to 16 which falls short of the 18-electron rule by 2.

 

So, either metals can bond multiple times is such complexes (such as a triple bond between the Fe ions which doesn't seem right to me at all) or is it due to bridging atoms in the complex - do bridging atoms supply less electrons to the metal in the complex?

 

Please can anyone help?

 

Many thanks,

Gem

[Comandante]

I'm new to Coordination Chemistry but I'm sure you can have less than 18 electrons. If you're unsure as to why this may be the case this section might help you as it specifically addresses that question: http://en.wikipedia.org/wiki/18-Electron_rule#Deviations_from_the_18-electron_rule