Transition metal ligand contributions, electron counts

Finally, just as BH3 coordinates to CO, so do a number of the higher borane cages. For example, B2H4(CO)2 (63), B3H7CO (64), B4H8CO (65), and BioHio(CO)2 (66) are all well characterized compounds. Because each boron atom with a coordinated CO contributes 3 electrons to the cluster count, only a few such exo-ligands can be accommodated. Dispite this, these clusters also express the close relationship between boron and transition metal clusters. 

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