Cluster nickel hydride

Schwarz s group has employed in-source CID to generate a range of metal hydrides for reactivity studies [78-80]. For example, the influence of additional ligands (L = CO, H2O, HO) on the reactivity of nickel hydride cluster cations, NiH toward methane has been explored (Eq. (6.40)) [78]. All systems react 3vith methane via a metathesis reaction (Eq. (6.40)), albeit at a slower rate than NiH+. A related... 

The proximal cluster donates an electron to the external acceptor. Two electrons are transferred from the hydride, at first reducing the nickel from Ni(III) to Ni(I). An electron is transferred from the nickel to the proximal cluster. 

A similar reaction can be written for the [Fe] hydrogenases with a Fe-[4Fe-4S] complex replacing the nickel. Note that the nickel atom in the NiFe cluster, and the Fe-[4Fe-4S] sites are nearest to the electron carrier [4Fe-4S] clusters, indicating that electron transfer occurs through these atoms. The other atom in each of the centres is an iron atom with -CN and -CO ligands, and it seems likely that this is a binding site for hydride (Fig. 8.1). 

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

Mean cobalt-cobalt and nickel-nickel distances observed in these complexes are very close to interatomic distances determined at ambient temperatures in cobalt and nickel metals (Co-Co 2.489(7) A vs. 2.507 A in a-cobalt (33) Ni-Ni 2.469(6) A vs. 2.492 A in the metal (39)). The mean M-H bond lengths, as well as hydride displacements from M3 faces, are less for nickel in H3Ni4(Cp)4 than for cobalt in HFeCo3(CO)9(P(OMe)3)3. Although the differences are marginally significant within error limits (Ni-H 1.691(8) A vs. Co-H 1.734(4) A displacements from plane Ni3 0.90(3) A vs. Co3 0.978(3) A), they are in the expected direction since the covalent radius should vary inversely with atomic number within a transition series. However, other effects such as the number of electrons in the cluster also can influence these dimensions. 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

Complete hydrogenation of isocyanides has not yet been achieved with these osmium compounds although it occurs readily with the nickel cluster Ni4(CNBu )7 (449). In the osmium case, however, the two different hydride... 

Binding energy, pentacarbonyliron, 6, 3 Binuclear complexes bis-Cp titanium halides, 4, 522 with Ni-M and Ni-C cr-bonds heterometallic clusters, 8, 115 homometallic clusters, 8, 111 Binuclear dicarbonyl(cyclopentadienyl)hydridoiron complexes, with rand C5 ligands, 6, 178 Binuclear iridium hydrides, characteristics, 7, 410 Binuclear monoindenyl complexes, with Ti(IV), 4, 397 Binuclear nickel(I) carbonyl complexes, characteristics, 8, 13 Binuclear osmium compounds, with hydrocarbon bridges without M-M bonds, 6, 619... 

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. 

The abundant chemistry of Ni(CO>4 under reductive reaction conditions leading to the formation of dinuclear nickel complexes or even to nickel clusters suggests the involvement of higher aggregates, however. An overview of the reactivity of nickel complexes, and of Ni(CO)4 in particular, is given in a series of excellent reviews by Jolly [13]. There seems to be evidence of an autocatalytic cycle for the formation of the active catalyst [14]. Parallel to this, the water-gas shift reaction (eq. (5)) occurs, resulting in the formation of carbon dioxide and hydrogen, which is known to form metal hydrides in the presence of metal carbonyls [15]. 

The system was very sensitive to the transition metal ion added to the platinum. When it was nickel, hydrocin-namaldehyde was obtained in 97% selectivity.75 The intermediates in these reductions are probably metal hydride clusters. When the colloidal platinum is supported on magnesium oxide, without another transition metal, the reduction produces the unsaturated alcohol with 97% selectivity.76 A rhodium colloid stabilized by the same polymer was used with a water-soluble phosphine in the hydro-formylation of propylene to produce 1 1 mixture of /r-bu-tyraldehyde and isobutyraldehyde in 99% yield.77 It could be used at least seven times, as long as it was not exposed to air. 

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