Hydrides, bridging ligands

Similarly, the low-temperature - C and H NMR spectra of [HFe4(CO)i3] indicate the presence of two distinct metal frameworks - a butterfly (4) and a tetrahedral core (5). The first contains a dihapto CO ligand bridging the wingtips whereas the hydride ligand bridges the hinge. 

Dimer in which the hydride ligands bridge the two metallic atoms. In the monomer, the metal is a Lewis acid because of the low number of d electrons (the valence layer is incomplete) there is thus a strong tendency to accept the doublet of a M-H bond of another identical molecule. 

The details for the synthesis of the activated tetraosmium hydrido cluster [Os4(/r-H)4(CO)io(NCMe)2] 72 have been previously reported the X-ray structure has now been determined. It consists of a tetrahedral metal core with each hydride ligand bridging the four OsOs bonds which are longer (2.915(3)-2.998(3) A) compared to the remaining two (both 2.810(3) A). The two acetonitrile ligands are coordinated to the same osmium atom (OsN = 2.02(4) and 2.07(5) A). The molecule possesses approximate symmetry. 

Rh4 core shows substantial distortion from a regular tetrahedron, resulting in four long and two short Rh—Rh distances. The four hydride ligands bridge the faces of the Rh4 cluster at a distance of 0.957 A above each plane formed by the three Rh atoms. Each C5 plane is 1.838 A from the rhodium to which it is bonded. ... 

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