Bonding in Hydride Cluster Compounds

From the point of view of bonding, terminal H-ligands form two-center/two-electron bonds meanwhile edge-bridging and face-bridging H-ligands are 

Formally three-center M-H-M bonds can be considered as formed by the donation of an electron pair from the metal-metal bond to the proton. 

Analogously, the bonding of a jU3-hydrogen to a triangular face of the cluster corresponds to the donation of an electron pair from a face-localized orbital to the l5 orbital of the proton. Clusters with face-bridging hydrogen ligands will have the same number of valence electrons than the parent anion. 

Unsaturated species such as the cluster [H4Re4(CO)i2] with 56 valence electrons can be rationalized by considering a parent hypothetical anion [Re4(CO)i2] with four face-delocalized, three-center/two-electron metal-metal bonds, each of which is able to coordinate one proton in the hydride. 

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