Physical Properties of the Metal-Hydrogen Bond

These studies have shown that the hydrogen atoms in metal-hydride complexes occupy accepted coordination positions, and the most reliable 

The structure determination on rans-PtHBr(PEt3)2 (228) (Fig. 1), which does not locate the hydrogen, indicates an approximately T-shaped configuration for the other ligands about the platinum, and the hydrogen is assumed to occupy a position trans to the bromine atom. 

Recent structural determinations on RhH(CO)(PPh8)3 by X-ray 196, 196a) and on potassium enneahydridorhenate, Ka[ReH9], by X-ray 191, 19S) and neutron diffraction 1), have located the hydrogen and have provided approximate metal-hydrogen distances, Rh—H = 1.60 0.12 196a) Re—H = 1.7 A (Figs. 4 and 5, respectively). 

The structure of the rhodium complex has no features attributable to a high rans-effect of the hydride ligand. For the salt K2[ReH9], there are three formula weights per unit cell 192), and the rhenium atoms are separated by large distances (minimum of 5.5 A) so that discrete enneahydridorhenate anions must exist in the crystal. The anion has Da, symmetry with six hydrogens at the comers of a trigonal prism and three beyond the centers of each equatorial face. The Re—H distance varies between 1.6 and 1.72 A 

Finally, an X-ray diffraction study on bis(jr-cyclopentadienyl)molyb-denum dihydride 21, 88a) indicates the structure shown in Fig. 6. The reported Mo—H distance of 1.1 0.2 A is rather shorter than might be expected in view of M—H distances in the above complexes, and this result needs confirmation. It is interesting to note that in the complex (7r-C5H6)2MoH2 the C6H6 rings do not have fivefold symmetry. 

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