Molecular hydrogen, coordination

The mechanism begins with the dihydride Ir(III) complex 127, and in the first two steps, the olefin coordinates in the trans position with respect to the phosphoms and molecular hydrogen coordinates in the axial position. Simultaneously, the olefin next undergoes a migratory insertion into the axial Ir-H bond, and H2 undergoes oxidative addition. The resulting species 130 is extremely labile with... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

Figure 2. IR spectra showing the substantial wavenumber shift between bands assigned to the V(H-H) vibration of coordinated molecular hydrogen in Mo(CO)5(H2), CrlCOj lH ), and W(CO) (H) in liquid Xe at -70°C. Note that the three traces were recor in separate experiments and that the absorbance scale refers to the Mo trace [28].

Kubas, G.J., R.R. Ryan, B.N. Swanson, P.J. Vergatini, J. Wasswerman, Molecular hydrogen complexes Coordination of a sigma bond to transition metals.. Am. Chem. Soc. 120,1988. 

In none of the cases discussed above is molecular hydrogen involved. The first report of the stoichiometric reduction of coordinated carbon monoxide by molecular hydrogen is that published by Bercaw et al. (35, 36). They reported that mononuclear carbonyl and hydride complexes of bis(pentamethylcyclopentadienyl)zirconium are capable of promoting stoichiometric H2 reduction of CO to methoxide under mild conditions. Thus, treatment of the dicarbonyl complex (rj5-C5Me5)2Zr(CO)2 with... 

This reaction is significant in terms of Fischer-Tropsch chemistry, because it represents the first well-characterized system in which a coordinated carbonyl is reduced by molecular hydrogen. Furthermore, complex 11 could be viewed as a precursor to ethylene glycol which, as previously indicated, is a highly desirable product from the reaction between carbon monoxide and hydrogen. 

Details of the various steps which will depend on the substrates and donors involved, are usually not well understood. Prior coordination of the donor followed by that of the substrate, equivalent to a hydride route (Section II,A), is also possible (494, 496). Formation of intermediate dihydrides from a donor (e.g., from an alcohol via oxidative addition to give a hydrido-alkoxide, and then /8-hydrogen transfer) has also been invoked (491, 492, 496, 499, 500) in mechanistic terms, the hydrogenations then become equivalent to using molecular hydrogen for the reductions. The /3-hydrogen transfer step is usually considered rate-determining (494, 496). 

A) Dative Hi coordination. Metals with empty orbitals can accept charge from the donor molecular hydrogen. Alternatively, this... 

RhCl(PPh3)3 oxidatively adds molecular hydrogen to form a five-coordinate dihydride complex which is consistent with the previous understanding [82] for olefin hydrogenation. This was also confirmed by experiments conducted by Mohammadi and Rempel [83] for NBR hydrogenation, where at 65 °C under 1 bar H2 the reaction is quantitative towards formation of the dihydride ... 

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