Acidic H2 complex

The first direct observation of equilibrium between an acidic H2 complex and a corresponding hydride complex with a protonated ancillary ligand is shown in Eq. (16) (70). 

Heterolytic activation of H2 on electrophilic complexes is important in catalytic hydrogenation and dates back 70 years.More recently, electrophilic activation of H2 has been found to be valuable in ionic hydrogenation where a mixture of an organometallic hydride, for example, CpMoH(CO)3, and a strong acid, for example, HO3SCF3, reduces sterically hindered olefins to alkanes via protonation to carbocations followed by hydride transfer from the metal hydride. It is likely that an acidic H2 complex is involved in the proton-transfer step (Scheme 7). 

The binding of dihydrogen to a metal center can lead to highly acidic -H2 complexes. The most acidic of these complexes are generated by protonation of a hydride complex by strong acid. A few of these complexes have been prepared... 

COORDINATIONCOMPOUNDS](Vol7) cis-Tetracaibonyl (2-3-h2-maleic acid) iron complex... 

Thiohydrates were regarded for a long time as the only H2S complexes which could be isolated. They are formed by addition of H2S to Lewis acids in liquid hydrogen sulfide (for example A1C13 H2S and TiCl4 H2S, n = 1, 2) and decompose thermally below room temperature. To stabilize complexes with the neutral ligands H2S and RSH, metal ions in low oxidation states, preferably with inert and bulky units, are required. 

However, when H2 is bound to a highly electrophilic cationic metal center, the acidity of H2 gas can be increased spectacularly, up to 40 orders of magnitude. The pK of H2 can become as low as —6 and thus the acidity of rf -H2 becomes as strong as that in sulfuric or triflic acid. As discussed in reviews by Morris (4,5) and Jia (25) and further work by Morris (26,27), such pK values are usually determined by NMR measurement of the concentrations of M H2 complexes in equilibrium with an external base such as a phosphine or amine. Electron deficient cationic and dicationic H2 complexes with strong short H-H bonds (<0.9 A) and weakly bound H2 such as [Cp Re(H2)(CO)(NO)]+ and [Re(H2)(CO)4(PR3)]+ are among the most acidic complexes (Table I). These acidic complexes typically have relatively high values of Jhd for their r 2-HD isotopomers, although pK values do not correlate... 

Reported piTa Values (Pseudo Aqueous scale) and Corresponding Jhd of Selected H2 Complexes, Emphasizing Highly Acidic Species... 

Several other examples have since been reported, including hydrogenation of alkynes and ketones (50). It is likely that an acidic H2 (or dihydride) complex is involved in the proton transfer step of some of these reactions [Eq. (11)]. 

Sellmann s model is claimed to be consistent with the severe constraints imposed on this N2-dependent HD formation from D2 and protons. Other modeling studies have shown that protons can be transferred from acidic H2 ligands in cationic Ru-H2 complexes to N2 ligands in W(N2)2(P)4 complexes (P = phosphine donor), in some cases even forming ammonia [Eq. (22)] (88,89). 

The protonation of a thiolate donor, formation of a nonclassical r 2-H2 complex, release of H2 and addition of D2, and the heterolytic cleavage of this D2 by the concerted attack of the Lewis acidic metal center and the Brpnsted basic thiolate donor are essential steps. The acidic thiol deuteron can exchange with EtOH protons. The resulting free protons and the deuteride complex yield HD and the coordinatively unsaturated species that is the actual catalyst. The detailed mechanism comprises a considerably larger number of steps (and equilibria) (143). For example, the occurrence of r 2-D2 and [M(D)(SD)] intermediates that exchange with H+ should give rise to [M(D)(SH)]... 

Finally, it may be noted that rf-H2 complexes can display acidity.88 While the pK of H2 itself is estimated to be ca. 35 in THF, coordination can drastically lower this. For example, frans-[Os(H2)(dppe)2(MeCN)](BF4)2 is markedly acidic (pKa = -2), dissociating to give H+ and [OsH(dppe)2(MeCN)]+.89... 

Carbonyl hydrides tend to be acids because of the stabilization of the resulting anions by the strong itt-acceptor power of the CO ligand (equation 20). H2 complexes can also be relatively acidic, because they are only formed when... 

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