Neutron diffraction metal hydride complexes

Figure 5.14 Neutron diffraction structure of the transition metal hydride complex (Me3P)4lr(OH)H. (Reproduced with permission from ref. 19.)...

Neutron Diffraction Studies of Tetrahedral Cluster Transition Metal Hydride Complexes HFeCo3(CO)9-(P(OCH3)3)3 and H3Ni4(C5H5)4... 

Prior Neutron Diffraction Work on Transition Metal Hydride Complexes... 

Of the monomeric metal carbonyl hydrides [HMn(CO)s,H2Fe(CO)4,HCo(CO)4], only HMn(CO)s has been analyzed crystallographically 10 4S). The lack of results for H2Fe(CO)4 and HCo(CO)4 is probably related to the fact that the compounds are thermally unstable. The neutron diffraction analysis of HMn(CO)s by La Placa and co-workers45) produced a Mn—H distance of 1.60(2) A. This result, together with the earlier measurement of the Re—H distance of 1.68(1)A in K2ReH9, provided conclusive proof that M—H distances in metal hydride complexes are normal (i.e., are consistent with the known covalent radii of the elements), and not anomalously short as suggested by some researchers (this controversy is reviewed in Refs. 1—3). 

In 1970, Frenz and Ibers reported the results of four neutron diffraction experiments in their structural review ). Since that time, over four dozen metal hydride complexes have been studied by this technique (see following tables). Based on these and supplemental X-ray results, some generalizations can be tentatively formulated ... 

It isn t always possible to get single crystals of metal hydride complexes suitable for single-crystal neutron diffraction smdies, nor do all metal hydrides give useful NMR spectra. What factors limit the apphcation of these methods In such circumstances, what else could you do to find out about the coordination of hydrogen ligands to the metal atoms ... 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

In addition, for metal complexes of the larger boron hydrides, more complicated types of bonding (where M-H-B linkages are augmented by M-B, M-M or B-B bonds) are found. As mentioned earlier in this article, metalloborane complexes are generally unsuitable for neutron diffraction because of the very large absorption cross... 

Hydrides. According to X-ray and neutron diffraction and metallographic studies of the Nb-H system,524 the H may be considered a lattice gas with phase transitions. In the a-, a -, j6-, and -phases of the system, H occupies tetrahedral interlattice positions. Whereas direct reaction between niobium metal and hydrogen occurs only after repeated activation of the metal by hydrogen absorption at ca. 7 atm and 350 °C, NbH2 is formed at temperatures as low as 22 °C in mixtures of LaNi5H6 7 and Nb.525 The extraordinary catalytic effect of the lanthanum-nickel complex is attributed to the presence of surface-absorbed atomic hydrogen species which are able to diffuse into the niobium lattice. There has been a review of the T a-H system.526... 

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