Neutron diffraction polyhydrides

One of the first metal polyhydride complexes to be thoroughly studied was ReHg, the nature of which was established firmly only after a neutron diffraction study reported in 1964 by Abrahams, Ginsberg, and Knox (18). At that... 

NMR spectroscopy and neutron diffraction studies established the tautomeric formulation of many polyhydrides as dihydrogen or H2) complexes or Nonclassical... 

A remarkable property of the Re-H bonds in polyhydrides is their ability to establish a new kind of hydrogen bond, M-H- HX (X = O, N, C), called protonic-hydridic or dUiydrogen bonds. Initially found in intramolecular systems, they can also be formed intermolecularly as established by neutron diffraction of the adduct ReH5(PPh3)2-C8H6 NHCeHe formed in the cocrystallization of the hydride with indole in benzene. ... 

Table 1 A selection of H... H distances in dihydrogen (f-ll2) and polyhydride (H)n complexes determined by neutron diffraction ...

The determination of and dMH both accurately and precisely is nearly always a challenge. In certain cases, especially in polyhydride complexes, there is ambiguity as to whether H2 ligands (elongated or otherwise) are really present, even in neutron diffraction structures. For example, as discussed in Section 4.8, [OsH5(PPhMe2)3]+ was originally formulated as an H2 complex,4,5 then calcula-tionally as a pentahydride, until finally a neutron diffraction study at 11 showed that it is indeed closer to a pentahydride with inequalities in dm (1.49 (HY3-HY4),... 

There has been mounting discussion on the criteria for evaluation of H—H distances and the formulation of fluxional polyhydrides as classical vs. nonclassical. It was pointed out that the predicted H—H bond distances from NMR Tj relaxation values must be corrected for relaxation effects due to H2 ligand rotation. This was supported by neutron diffraction data. Crabtree has suggested that temperature-dependent minimum Tj times be used to identify molecular... 

It forms yellow crystals which lose hydrogen readily it must therefore be kept under hydrogen. The structure, determined at — 100°C by X-ray difiraction and at room temperature by neutron diffraction, shows that direct H—H bonding is retained. Moreover, in the n.m.r. spectra of the HD complex, coupling between and nuclei is observed (J d = 33.5 Hz cf. HD( ), 43.2 Hz). Protonation of the polyhydride IrH5L2 (L = PCy3) affords a cationic species in which two H molecules are bonded to the metal centre. 

Potassium nonahydridorhenate(VII) K2[ReH9] was prepared by reduction of potassium perrhenate in aqueous ethylenediamine with potassium or lithium metal. Neutron diffraction analysis established unequivocally the correct composition and molecular structure of this polyhydride complex. The diffraction study was performed within an evacuated cryostat maintained at room temperature, since the crystals are unstable in moist air. The Re atom hes at the center of a trigonal prism of H atoms, with three H atoms beyond the centers of the prism faces. Thus, the structure is a tricapped trigonal prism of symmetry. The average Re—H distance is 1.68(1) A and the average H—Re—H angle between hydrogens in the same vertical mirror plane is 93.6(6)°. 

Another example of the nine-coordinate polyhydride is WHg PPh(i-Pr)2 3 which was prepared by subjecting a hexane solution of hexamethyltungsten and PPh(i-Pr)2 to hydrogen at 300 atm for five days. The molecular structure was also confirmed by neutron diffraction... 

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