Structural and NMR Studies of Dihydrogen Complexes

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), 

10 an unsatisfactory answer to a critical question. The solid state NMR dHH for the unsolvated complex is 0.88 A, which is typical in octahedral d6 complexes with CO or hydride trans to H2. Thirteen complexes, mostly octahedral d6 complexes, have been studied by neutron diffraction, only three of which have dHH corrected for libration. 

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