H2 complex

In a sense the formation of t) -H2 complexes can be thought of as an intermediate stage in the oxidative addition of H2 to form two M-H bonds and, as such, the complexes might serve as a model for this process and for catalytic hydrogenation reactions by metal hydrides. Indeed, intermediate cases between and... 

Tn the absence of Eu oxidative depro-tonation of the pale-yellow H2S complex occurs to give the orange ruthenium(TTT) complex [Ru(NH3)5(SH)]. Other examples of complexes containing the SH ligand are [Cr(OH2)5(SH)f+, [W(f, -CsHs)(CO)3(SH)],... 

Characteristic infrared absorption lines have been identified for various hydrogen-acceptor and hydrogen-donor complexes (see Chapter 8), and the strength of such a line in any given specimen is a measure of the quantity of the complex present. However, depth resolution is crude, and masking by free-carrier absorption is sometimes a problem. Raman lines have also been seen (see Chapter 8) and in principle should be capable of detecting species that are not infrared active however, the sensitivity is low, and the most interesting and presumably abundant species, an H2 complex, has not yet been detected in this way. 

The final question we shall consider here has to do with the extrapolation of the solubility of hydrogen in silicon to lower temperatures. Extrapolation of a high-temperature Arrhenius line, e.g., from Fig. 11, would at best give an estimate of the equilibrium concentration of H°, or perhaps of all monatomic species, in intrinsic material the concentration of H2 complexes would not be properly allowed for, nor would the effects of Fermi-level shifts. Obviously the temperature dependence of the total dissolved hydrogen concentration in equilibrium with, say, H2 gas at one atmosphere, will depend on a number of parameters whose values are not yet adequately known the binding energy AE2 of two H° into H2 in the crystal, the locations of the hydrogen donor and acceptor levels eD, eA, respectively, etc. However, the uncertainties in such quantities are not so... 

The conclusion (ii) has important implications for the binding energy of H2 , though further assumptions have to be made to draw specific conclusions. If H2 is indeed a diatomic H2 complex, the dominant dissociation process for this species in the region of the small step of Fig. 25 is presumably... 

Dative and synergistic complexation of dihydrogen Direct observation of complexation of molecular H2 at metal centers is a relatively recent phenomenon. Virtually all such complexes exhibit relatively minor elongation of the H—H bond, indicating that description of these species as molecular-H2 complexes (rather than, e.g., metal dihydrides or some intermediary resonance mixture) is well justified. Experimentally observed molecular H2 complexes of the synergistic type are common, but those of simple dative type are not. 

As shown in Fig. 4.69, the HfFLi- alkene complex exhibits expected parallels with the HfFLi- H2 complex (Fig. 4.59), both in terms of molecular shape and in terms of valence interactions. The characteristic features of such weak dative bonding include long Hf—C distances (2.82 A), normal C=C bond length (1.34 A), planar alkene bond angles, and small binding energy (15.1 kcalmol-1)-... 

The mechanism is dominated by the remarkable stability of the Fe( 72-H2) bond, which is one of the most stable 72-H2 complexes reported in the literature [8, 10]. Remarkably, the free coordination site for the incoming alkyne is provided by the reversible dissociation of one of the phosphine moieties of the PP3 ligand rather than dissociation of the dihydrogen ligand (see Scheme 14.1). The coordinated alkyne subsequently inserts into the Fe-H bond and the emerging Fe-vinyl bond is... 

Supercritical fluids allow the formation of species that cannot be made in conventional solvents. For example, rj2-H2 complexes have been generated by direct reaction of hydrogen with a transition metal carbonyl complex [10]. In order to isolate these compounds, a continuous flow reactor was used and such compounds could be isolated with surprising ease. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Thus the two plausible catalytic cycles have been considered, one via an Ir dihydride complex A and the other via an IrH2(ri -H2) complex B (Fig. 3). The first is analogous to the well-established mechanism for rhodium diphosphine-catalyzed hydrogenation of olefins going through Ir(I) and Ir(III) intermediates [26-29]. 

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