Metal hydride bonding models

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

Further evidence for the validity of the frontier orbital approach derives from its success in predicting the shift (increase or decrease) in naked cluster IP upon the chemisorption of small reactant molecules. For all metal clusters examined thus far, H2 chemisorption induces an increase in cluster IP. ° This follows directly from interactions (1) and (2), since the creation of the two new metal-hydride bonding orbitals effectively removes two electrons from the cluster valence orbital manifold. Thus with resjiect to the metal cluster, H2 chemisorption can be viewed as an oxidative addition process. If a one-electron (Aufbau filling) approximation is assumed as above, the Fermi level of the cluster is shifted toward lower energy, that is, there is an increase in IP. As the cluster grows larger, the shift in IP diminishes. This is simply a manifestation of cluster-size-dependent variations in the valence orbital density of states, and is again consistent with the frontier orbital model. 

Hydridometal complexes are simple model systems for a cofactor-cnzyme biological assembly. The metal hydride provides a two-electron center, and the reactivity of this species allows the insertion of various substrates into the metal-hydride bond. Thus this reactive species can be considered as an active site, where two ET transformations can proceed. Several transformations that can be driven by such a pathway are summarized as follows ... 

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... 

As idealized computational models of metal hypovalency, let us therefore consider the early second-series transition-metal hydrides YH3, ZrH4, and NbfL (avoiding both the complications of lone-pair-bearing ligands and those associated with the lanthanide series). Figure 4.54 shows optimized structures of these species, and Table 4.33 summarizes the bonding (omh) and nonbonding (nM ) orbitals and occupancies at the metal center. 

A variety of factors contribute to the great current interest in l polynuclear metal hydride complexes. These include the novel geometries found in these systems and their usefulness as models for the bonding of hydrogen to metals, such as may occur in catalysis (i) or hydrogen-storage applications (2). A comprehensive review of metal hydride complexes, in which polynuclear species are included, has been published by Kaesz and Saillant (3). 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

Facile isocyanide insertion reactions into metal-carbon, -nitrogen, -sulfur, -oxygen, - hydride, and - halide bonds have been found to readily occur. The insertion into metal-hydrides to give stable formimidines is particularly noteworthy since corresponding formyls (—CHO) are exceptionally difficult to synthesize and tend to be very unstable. There is a great deal of interest in carbon monoxide reductions, and the instability of the intermediate reduction products has made a study of the reduction process extremely difficult. Recently, however, the interaction of isocyanides with zirconium hydrides has allowed the isolation of the individual reduction steps of the isocyanide which has provided a model study for carbon monoxide reduction (39). 

The presence of H- in metallic lanthanide hydrides was proposed by Dialer (9), and the idea later extended to other metallic hydrides (11, 23). As at present interpreted the model suggests that H is associated with a helium-like configuration of electrons, in a rather low-density electron sea. The metal is considered to be the inert-gas core (or a stable non-inert-gas core) with a localized net positive charge derived from the stoichiometry of the hydride. The remaining electrons of the metal are considered to be in the usual directed hybrid orbitals, whose directions help determine the crystal structure and whose bonding to nearest metal neighbors stabilizes the structure. Electrons in these directed orbitals are sufficiently delocalized to provide a conduction band and metallic or semimetallic properties. [Compare the model of TiO and VO proposed by Morin (29).]... 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

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