Hydride Hydrido-bridged cations

The p-hydrido bridged cation [90] is equivalent to a resonance system such as exists in allyl cations (single energy minimum) and can be distinguished from a set of two degenerate cycloalkyl cations [88] undergoing a fast hydride shift (double energy minimum) as shown in (70) by comparison with suitable model cations. 

The rearrangement of [13] bears a close resemblance to the transannular reactions observed in medium sized rings that have been reviewed by Prelog and Traynham (1963) and Cope et al. (1966). Recently Sorensen and coworkers have studied medium sized cycloalkyl cations under stable ion conditions in non-nucleophilic media and demonstrated that their structures are ji.-hydrido-bridged. The bonding situation in these ions contrasts sharply with that in the ions described above and rather corresponds to that of transition states (or intermediates) for intramolecular hydride transfer in these ions. 

The bidentate oxazoline ligands 85 and 86 (and derivatives thereof) are excellent reporter ligands, and several studies have used NOEs to determine the nature of their chiral pockets [61, 113, 114, 126]. NOESY studies on the cations [Ir(l,5-COD)(86)]+ and several cationic tri-nudear Ir(iii)(hydrido) compounds [110], e. g. [Ir3(p3-H)(H)5(86)3] +, 87, in connection with their hydrogenation activity, allowed their 3-D solution structures to be determined. In addition to the ortho P-phenyl protons, the protons of the oxazoline alkyl group are helpful in assigning the 3-D structure of both the catalyst precursors and the inactive tri-nudear dusters. Specifically, for one of these tri-nudear Ir(iii) complexes, 87 [110], with terminal hydride ligands at d -17.84 and d -21.32 (and a triply bridging hydride at 5 -7.07), the P-phenyl and oxazoline reporters define their relative positions, as shown in Scheme 1.5. 

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