Hydride fluxionality processes

Reactions leading to a series of remarkable dizirconium alkyls and hydrides are outlined in Scheme 213. In solution, the tetrahydride undergoes a rapid fluxional process (observed by NMR spectroscopy) that exchanges the four hydrides. At low temperature three distinct hydrides can be observed, which is consistent with either a dibridged structure, or a tri-bridged species undergoing... 

A common feature of metal clusters is their stereochemical nonrigidity, in which carbonyl and hydride ligands exchange their coordination sites. Mixed-metal clusters are ideally suited for studies of the fluxional processes in clusters because of the low symmetry inherent in their metal framework. In such clusters, the majority of the ligands are in chemically nonequivalent positions and should thus be distinguishable by NMR... 

As usual, H- H coupling is lost in the broad lines of boranes. The bridging hydrides are seen as the broad unresolved multiple centred at -1.38 ppm. The width of this signal may be associated with B-B coupling (a second-order effect) or with some fluxional process. It is typical of bridging B-H-B resonances. The three remaining terminal hydride signals should each appear as a quartet as a result of... 

Fluxional processes in agostically stabilized 14e alkyliridium hydride complexes have been studied by means of DFT computations, see N. Philipps, L. Treasure, N. H. Rees, R. Tirfoin, J. E. McGrady and S. Aldridge, Eur. J. Inorg. Chem., 2014, 4877-4885. 

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