Protonation of metal hydrides

Addition of protons often occurs readily to a variety of neutral and anionic complexes and is a rich source of hydrides (equation 12). Protonation can also take place on a ligand in certain cases. Protonation of metal hydrides is a common source of H2 complexes (equation 13). 

Kinetic studies of the protonation of metal hydrides with acids to form H2 complexes of the type [FeH(H2)P4]+ (P4 = 2dppe or P(C2H4PPh2)3) show first-order dependence on concentration of both complex and acid.32 This reaction in THF involves direct attack of acid at one hydride in FeH2P4, apparently by the mechanism in Scheme 5.4, where a type of hydrogen bonding interaction ( dihydrogen bonding ) is likely to be the first step. 

The EIE values for H2 versus D2 addition are usually inverse over a large temperature range (260-360 K), showing that counterintuitively D2 binds more strongly than H2. The values of Kh/Kd observed thus far are 0.36-0.77 for formation of H2 complexes and 0.47-0.85 for complete OA. An inverse EIE (0.39) also occurs for protonation of metal hydrides to form T 2-H2 as shown in the reaction Cp2WH2 I H 1 [Cp2WH(H2)] +. 191... 

Using the information discussed above, protonation of metal hydrides can now be considered as going via the following pathway (Equation 4). 

Several synthetic routes to H2 complexes are available. The simplest method is reaction of H2 gas with a coordinatively unsaturated complex such as W(CO)3(PR3)2, the original method, as described in Chapter 2. This is analogous to the well-studied reversible addition of H2 to Vaska s complex, IrCl(COXPPh3)2, except that for the latter OA to the dihydride occurs. A second, very common method of preparation is protonation of metal hydride complexes ... 

Kinetic Protonation of Metal Hydrides to Give Dihydrogen Complexes... 

Thus dihydrogen complexes play a key role in the evolution of gas after protonation of metal hydrides. The reverse of this process, heterolytic activation to form and a metal hydride, can be used to hydrogenate polar double bonds. 

Protonation of metal hydrides may often go via the pathway of Eq. 2. which also shows the side-on binding of the H2 ligand resulting from proton transfer. 

Although many different synthetic routes have been described, the three major routes [10b] leading to H2 complexes seem to be (1) addition of H2 to unsaturated 16-electron complexes (2) photolytic displacement of CO (3) protonation of metal hydrides. 

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