Terminal Hydrides

Several side reactions or post-cuting reactions are possible. Disproportionation reactions involving terminal hydride groups have been reported (169). Excess SiH may undergo hydrolysis and further reaction between silanols can occur (170—172). Isomerization of a terminal olefin to a less reactive internal olefin has been noted (169). Viaylsilane/hydride interchange reactions have been observed (165). 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

IV. Terminal Hydride Derivatives of the s- and p-Block Metals Supported by Poly(pyrazolyl)borato Ligation... 

By analogy with the alkyl derivatives described earlier, poly(pyrazo-lyl)hydroborato ligation may also be utilized to support monomeric terminal hydride derivatives [TpRR ]MHn. However, at present, the chemistry of such complexes is effectively restricted to that of four-coordinate derivatives of the type [TpBut]MH. 

NMR data indicate a structure for the latter having a direct Pt-Pt bond with a terminal hydride bound to one Pt and the CH CN ligand to the other. 

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. 

Higher resolution measurements of the Si-H stretching band in model compounds revealed a significant difference in the frequency of this band between terminal and internally placed hydride groups (Figure 3). For terminal hydrides the frequency was about 2127 cm l, while for internal ones it was about 2167 cm l (Table I). The frequency for the two Sylgards was about 2163 cm l, nearly that for internal placements, and in line with a random distribution. 

The HNMR spectrum was recorded in a chloroform- solution. The hydride region of the spectrum shows two sets of multiplets, each with platinum satellites, corresponding to the bridging and to the terminal hydrides with the following parameters. 

The Ir(III) metal centres in the products, which are bound to a terminal hydride and a bridging —NH2 group, represented the first X-ray stnictural authentication of a transition metal species with both amide and hydride bound to a metal centre. The reactivity of the complexes is low, however, and appears to be dominated by the stability of the lr(p-NH2)2lr bridging unit. More recent work has shown that olefin-iridium(l) complexes, such as the propene species [ HC(CH2CH2PBu 2)2 Ir(CH2CHMe)], react diiectly with ammonia at room temperature as shown in Equation (6.12)." ... 

Whereas edge- and face-bridging hydrogen atoms are associated mainly with metal-metal orbitals already present in the tetrahedral skeleton (see last section) and therefore have limited steric requirements, terminal hydrides occupy a full coordination position. It is not surprising then that the only examples of tetrahedral species with terminal hydrides contain 11 and 10 carbonyls, [Ir4(CO)nH] and [Ir4(CO)ioH2]2 . (The assignment for [Ir4(CO)nH] is based on the structure determination of the analogous [Ir4(CO)nBr] and on the similarity of the ir spectra of the two species (11).)... 

In the penta-, hexa-, and heptanuclear carbonyl hydride clusters, terminal hydrides are observed only in the less crowded species, and again no evidence for the presence of interstitial hydride is found. 

Location of Terminal Hydride Ligands in Transition Metal Hydrides... 

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