Face-capping carbonyl ligands

Neutral and anionic organoimidoimido clusters of rhodium)I) with diolefin and carbonyl ligands are accessible and stable. The diolefin neutral compounds [Rh4(/i-N-/7-tolyl)2(diolefin)4] are a convenient entry into this chemistry for which the synthetic route is an apparently simple replacement reaction, but the reproducibility of the results requires a careful technique. These tetranuclear compounds contain a trimetallic triangular cluster face capped on both sides by two pnra-toly-limido ligands and a further rhodium fragment coordinated to a para-to y ring in a 7 -fashion, giving an overall valence electron count of 64e. 

The carbonyl ligand can also bridge more than two metals. The carbonyl often is a ligand bridging three metals (capping a triangular cluster face), and each CO gives 2/3 electron to each metal. 

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