Analyse other transition metal carbonyl complexes: does this effect occur for all metals?

•	This question is best answered by looking at the oxidation states for some of the complexes in our hitlist. What do you notice?

•	For the vast majority of complexes, the oxidation state is 0 e.g. BILMIS, OGAZAX. There are some complexes where the oxidation state is 2 e.g. FIRFUI, SONROC. The highest oxidation state is 4 (CECZEP, CECZIT). In short, all the complexes in our hitlist have fairly low oxidation states.

•	This is the case for metal carbonyls: for effective pi back bonding to occur there must be sufficient electron density on the metal. Metals in higher oxidation states will not have enough electron density to facilitate the back bonding.