Metal-H2 complexes

Since then nearly 600 metal-H2 complexes, representing every transition metal from vanadium to platinum, have been isolated or observed... 

Figure 8.2 Metal-H2 complexes explored by Kubas [38], Crabtree et al. [42], and Morris and...

The interaction between H2 molecules and exposed metal ions has been studied for decades. The initial motivation to study this type of interaction came from the activation and cleavage of hydrogen by metal centers for important chemical reactions. The discovery of metal-H2 complexes has enabled chemists to examine the coordination nature of the (t donors, such as a H2 molecule. The u-electrons of H2 can coordinate to a vacant metal d orbital, whereas its empty coordination mode of the molecule, where the H2 a and a bonds can fully interact with the metal d orbitals (Figure 3). 

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... 

One mole of the complex-forming H2 Y2 reacts in all cases with one mole of the metal ion and in each case, also, two moles of hydrogen ion are formed. It is apparent from equation (o) that the dissociation of the complex will be governed by the pH of the solution lowering the pH will decrease the stability of the metal-EDTA complex. The more stable the complex, the lower the pH at which an EDTA titration of the metal ion in question may be carried out. Table 2.3 indicates minimum pH values for the existence of EDTA complexes of some selected metals. 

Metal-polysulfido complexes have been synthesized by a variety of methods using various reagents as sulfur sources, e.g., Ss, M2S (M=alkali metal), P2S5, H2S, organic polysulfanes, etc. The nature of the resulting polysulfido complexes often depends on the reaction conditions such as the ratio of starting materials, solvents, reaction temperature, and reaction time. In addition, the use of different ligands leads to the different results in most cases. This section shows typical synthetic methods for metal-polysulfido complexes based on recent reports on their syntheses. 

In 1931, Hieber and Leutert reported Fe(CO)4(H)2 not only as the first iron hydride complex but also as the first transition-metal hydride complex (FeH2 was reported in 1929 from FeCl2 and PhMgBr under a hydrogen atmosphere. However, it exists only in a gas phase) [2, 3]. The complex synthesized from Fe(CO)5 and OH (Scheme 1) is isolable only at low temperature and decomposes at room temperature into Fe(CO)5, Fe(CO)3, and H2. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

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. 

However, while ruthenium carbonyl was found to decompose the formate ion in basic media, the rate was slower (<0.1 mmol trimethyl ammonium formate to H2 and C02 per hour) than the rate of the water-gas shift reaction (>0.4 mmol H2/hr) at 5 atm CO and 100 °C. Increasing CO pressure decreased the formate decomposition rate. However, it was observed that increasing the CO pressure from 5 atm CO to 50 atm increased the H2 production rate to 10 mmol/hr. They proposed, in a similar manner to Pettit et al.,34 a mechanism that involved nucleophilic attack by amine (instead of hydroxide). Activation of the metal carbonyl (e.g., Ru3(CO) 2 cluster to Ru(CO)5) was suggested to be favored by dilution, increases in CO pressure, or, in the case of Group VIb metal carbonyl complexes, photolytic promotion. The mechanism is shown below in Scheme 9 ... 

Bimolecular decomposition of a non-metal hydride organometallic complex, such as metal formyl complexes, which may involve metal hydride precursors MC( = 0)H + H20 <-> M(CO)+ + OH + H2 N/A 36... 

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