Thermodynamic data hydrides

Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (85). 

Both kinetic and thermodynamic data on organometallic hydrides should be very useful. The relative rates of proton transfer processes and other reactions determine a good deal of organometallic chemistry. For example, in our synthesis of cis-0s(C0) (CH )H> reactions 2-4, the comparative rates of... 

Thermodynamic data on the acidity of organometallic hydrides should help identify situations where apparent reactions of acidic transition-metal hydrides actually result from their conjugate bases. A case in which both species can react but give different products (as was pointed out by Prof. Espenson three years ago (18)) is the addition of hydridocobaloximes, HCo(dmgH)2B, to olefins with electron-withdrawing substituents... 

The insertion of CO is in many instances thermodynamically unfavourable the thermodynamically most favourable product in hydroformylation and carbonylation reactions of the present type is always the formation of low or high-molecular weight alkanes or alkenes, if chain termination occurs via (3-hydride elimination). The decomposition of 3-pentanone into butane and carbon monoxide shows the thermodynamic data for this reaction under standard conditions. Higher pressures of CO will push the equilibrium somewhat to the left. 

The spectroscopic, kinetic, and thermodynamic data discussed are sufficient to describe semiquantitatively the energy profile of proton transfer to a hydride ligand occurring in solution [29, 35, 36]. Figure 10.10 shows the energy as a function of the proton-hydride distance, varying from the initial state to a final product. The average structural parameters of the initial hydrides and intermediates have been taken from earlier chapters. Since proton-hydride contacts of... 

Pressure-composition-temperature and thermodynamic relationships of of the titanium-molybdenum-hydrogen (deuterium) system are reported. 0-TiMo exhibits Sieverts Law behavior only in the very dilute region, with deviations toward decreased solubility thereafter. Data indicate that the presence of Mo in the 0-Ti lattice inhibits hydrogen solubility. This trend may stem from two factors for Mo contents >50 atom %, an electronic factor dominates whereas at lower Mo contents, behavior is controlled by the decrease in lattice parameter with increasing Mo content. Evidence suggests that Mo atoms block adjacent interstitial sites for hydrogen occupation. Thermodynamic data for deuterium absorption indicate that for temperatures below 297°C an inverse isotope effect is exhibited, in that the deuteride is more stable than the hydride. There is evidence for similar behavior in the tritide. 

The oxidation reaction of water vapor with a metal to form the oxide and a hydride is also of interest, but no accurate thermodynamic data are available on hydrides of these metals. 

Some indications in the form of thermodynamic data are expected from a series of hydride-exchange reactions, e.g., with 1,3-dithiole (84) or tropilidene (85). In view of the large volume of numerical... 

Thermodynamic data of selected hydrides showing equilibrium pressure at 25 °C, and equilibrium temperature at 1 atm pressure are shown in Table 12.3 using the data of Sandrock [17]. Some selected alloys/intermetallics are shown that have favorable thermodynamic properties for practical application these properties of the alloys/intermetallics are listed in Table... 

Thermodynamic data showing hydriding and corresponding temperature for standard equilibrium conditions for the AB5 and AB2/A2B/AB type hydrides [97]. 

It has been argued that AA for most metal hydrides will approximately equal the entropy of the evolved hydrogen, c. 130JK mor (H2) (Ziittel et al., 2003), but complex hydrides can have a lower AA, e.g. 95 JK mor HH2) as can be determined for LiBH4 from standard thermodynamic data (Lide, 2002). Using these two values for AA one can estimate with equation... 

The thermal stability of the known hydrides varies widely from the stable tertiary phosphine complexes (whose decomposition temperatures are frequently above 250°C) to the notoriously unstable mononuclear carbonyl hydrides. Although there are no thermodynamic data, it seems probable... 

On the basis of the kinetic and thermodynamic data, a plausible mechanism for the Tishchenko reaction is presented in Scheme 15. In the first step of the reaction, the precatalyst 1 reacts with two equivalents of the aldehyde to give exothermically the alkoxo complex 42 (Step i in Scheme 15 AHcaic = —68 kcal/mol). A second insertion of an aldehyde into the thorium-alkoxide bond yields complex 43 (step ii in Scheme 15). The concomitant hydride transfer from complex 43 to an additional aldehyde releases the ester 44 and produces the active catalytic species 45 (step iii in Scheme 15). The insertion of an aldehyde into complex 45 (step iv, AHcaic = —25 kcal/mol) gives complex 46, and its hydride transfer reaction (step v, rate determining step, AHcaic = —22 kcal/mol) with an additional aldehyde via a plausible six-centered chair-like transition state (47) produces the ester 38 and regenerates the active complex 45. 

Hydrides. The phase diagram and thermodynamic data for the V-H system have been presented. 

There have been some speculations about the existence and transport of hydride ions (H ) in oxides under reducing conditions, but according to thermodynamic data of hydrides, the conditions for a hydrogen-separation membrane will be much too oxidizing for hydride ions to be stable [9]. Moreover, the apparent indications of hydride ions in the literature have now been rationalised by other, more credible phenomena, actually arising from transport of neutral hydrogen [2]. Therefore, it seems, at present, that hydride ions play no role in hydrogen permeation in oxides. 

The thermochemical cycle in Scheme 6 has been used to estimate the effect of a one-electron oxidation on the thermodynamic acidities of metal hydrides. The method has been similarly used on organic systems. Measurement of the oxidation potentials for the metal hydride and its conjugate base gives access to relative values for the metal hydride and its one-electron oxidized counterpart through Equation (14), Scheme 6. In many reported cases, one (or even both) electrode potentials are obtained from chemically irreversible voltammograms, with consequential uncertainties in the derived thermodynamic data. Table 7 gives a comprehensive list of M-H data comparing MH and MH species as determined with this thermochemical cycle. 

One fundamental aspect of carbocation chemistry is the large dependence of the energy of the cation on the substituents attached to the positively charged carbon atom. This dependence is most evident in the gas phase. Table 5.4 shows thermodynamic data for selected carbocations. The hydride ion affinity, HIA(R ), is defined as the negative of the AH for the attachment of a hydride ion to the cation in the gas phase. That is, the greater the H1A(R ), the more endothermic is the removal of a hydride ion from an alkane. We expect the trends to be the same for heterolytic dissociation of alkyl halides or other species that produce carbocations. 

TEMPO+ abstracted hydride anions from the hydrides of aldehydes and ketones in acetonitrile without any side products. The hydride affinity of aldehydes and ketones in acetonitrile was defined as fhe enthalpy change of the aldehydes and ketones. Several conclusions regarding the hydride-accepting abilities of aldehydes and ketones, based on thermodynamic data, were listed." " ... 

The thermochemical properties of the lanthanide-hydride systems have been well-catalogued by Libowitz and Maeland (1979). Heats, entropies and free energies of formation have been tabulated for both the dihydrides and dideuterides, as well as data for the La through Nd trihydrides and also the hydrogen-deficient hexagonal phases. Methods are described for typical calculations and are not repeated here. Thermodynamic data and thermal functions are presented by Flotow et al. (1984) and by Ward (1985a and b) for the hydrides of Th through Am. Newer data are reviewed here, and the values in table 1 reflect these updates. 

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