Diffraction transition 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... 

X-ray diffraction is a standard method employed by chemists to determine the structure of new complexes. In the case of transition metal hydride and dihydrogen complexes, precise location of metal-bound hydrogen atoms by X-ray diffraction is very difficult. Superior structural information is provided by neutron diffraction, but the requirement for large well-formed single crystals has limited this method to a small subset of the known complexes.118... 

Reactions between [Cp Ir(PMe3)(Me)(OTf)] and aldehydes (RCHO) proceed with high selectivity to give the hydrocarbyl carbonyl salts [Cp Ir(PMe3)(R)(CO)]OTf (137, R = Me, Et, Pr, Ph, 1-ethylpropyl,/>-Tol, Mes, (Z)-l-phenyl-l-propen-2-yl, vinyl, Bu, 1-adamantyl). The tandem C-H bond activation/decarbonylation reaction afforded the first isolated tertiary alkyl complexes of Ir. X-ray diffraction studies were carried out on Mes, Bu, and 1-adamantyl derivatives. Hydride reduction of the /)-Tol complex provided an example of a rare transition metal formyl complex, [Cp lr(PMe3)(p-Tol)(CHO)]. ... 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

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