Hydrides thermodynamics

Cobalt is invariably present in commercial MHt battery electrodes. It tends to increase hydride thermodynamic stability and inhibit corrosion. However, it is also expensive and substantially increases battery costs thus, the substitution of Co by a lower/cost metal is desirable. Willems and Buschow [40] attributed reduced corrosion in LaNi 5 vCoi (x= 1 -5) to low Vn. Sakai et al. [47 J noted that LaNi25Co25 was the most durable of a number of substituted LaNi5 iCoi alloys but it also had the lowest storage capacity. 

When we deal with the kinetics of hydride reactions we have to be aware that hydride thermodynamics cannot be properly formulated without taking into account the (relative) immobility of the metal component. This immobility can sometimes render the interpretation of the experimental reaction kinetics ambiguous. With this difficulty in mind, let us outline concepts which describe the kinetics of hydride formation and decomposition. An extensive account, including a first order phenomenological treatment, has been given by [P.S. Rudman (1983)]. The conceptual framework for a more rigorous discussion is found in, for example, [G. B. Stephenson (1988)]. 

In non-fluoride-containing solutions, silicon is stable due to the presence of an oxide film and the electrode behavior can remain constant under a continuous cathodic polarization. The surface of a silicon electrode in fluoride-containing aqueous solution at the open circuit potential is also stable due to hydrogen adsorption. However, surface transformation can occur at cathodic potentials due to formation of hydrides. Thermodynamically, silicon hydride can be a stable phase at certain cathodic potentials as shown in Fig. 2.2. 

Y-shaped, while 46-syn is best described as T-shaped. Computational modeling studies suggest that the structure of 46-anti is the electronically preferred one, whereas that of 46-syn results from distortions provoked by sterics. Such small structural differences lead to more significant differences in reactivity between syn and anti isomers. Thus, syn isomers of 38, 46, and 47 form six-coordinate adducts with chlorinated solvents, CO, and P(OMe)j after coordination of the incoming hgand trans to Si. The anti isomers do not form detectable adducts with chlorinated solvents and coordinate CO or P(OMe)j either trans to Si (kinetic product) or trans to hydride (thermodynamic product). The equihbrium distribution of isomers for such six-coordinate adducts is dependent on the nature of the hahde ligand, such that in the case of CO adducts, the replacement of chloride by iodide inverts the stereochemistry of the reaction product and switches the CO coordination position from trans to hydride (98%) to trans to Si (100%). 

In addition to the hydrides of formula HjX, oxygen forms the hydride H2O2, hydrogen peroxide, and sulphur forms a whole series of hydrides called sulphanes. These are yellow liquids which are thermodynamically unstable with respect to hydrogen sulphide and sulphur. 

Thermodynamic. Thermodynamic properties of Pu metal, gaseous species, and the aqueous ions at 298 K are given in Table 8. Thermodynamic properties of elemental Pu (44), of alloys (68), and of the gaseous ions Pu", PuO", PuO" 27 PuO 2 (67) have been reviewed, as have those of aqueous ions (64), oxides (69), haUdes (70), hydrides (71), and most other compounds (65). 

Because these stability measurements pertain to the gas phase, it is important to consider the effects that solvation might have on the structure-stability relationships. Hydride affinity values based on solution measurements can be derived from thermodynamic cycles that relate hydrocarbon p T, bond dissociation energy and electrochemical potentials. The hydride affinity, AG, for the reaction... 

Anhydrous HX are versatile and vigorous reagents for the halogenation of metals, non-metals, hydrides, oxides and many other classes of compound, though reactions that are thermodynamically permissible do not always occur in the absence of catalysts, thermal initiation or photolytic encouragement, because... 

Interestingly, true hydrides, such as NaH and KH, do not reduce carbonyl groups. Using energies of hydride and methoxide (at left), calculate AH xn for the reduction of formaldehyde by H. Is this reaction more or less favorable than those based on ZH4 Can the low reactivity of NaH and KH be attributed to thermodynamic factors, or must kinetic factors be responsible ... 

The intramolecular Michael addition11 of a nucleophilic oxygen to an a,/ -unsaturated ester constitutes an attractive alternative strategy for the synthesis of the pyran nucleus, a strategy that could conceivably be applied to the brevetoxin problem (see Scheme 2). For example, treatment of hydroxy a,/ -unsaturated ester 9 with sodium hydride furnishes an alkoxide ion that induces ring formation by attacking the electrophilic //-carbon of the unsaturated ester moiety. This base-induced intramolecular Michael addition reaction is a reversible process, and it ultimately affords the thermodynamically most stable product 10 (92% yield). 

Flanagan and Oates [1] have extensively reviewed the thermodynamics of intermetallic hydrides also recommended are the classic work of Libowitz [2] and the com-... 

Treatment of the optically active formaldehyde dithioacetal monoxide with ethyl benzoate in the presence of sodium hydride gives the benzoylated product as a diastereomeric mixture in a thermodynamically controlled (65 35) ratio66. 

Addition of the chelated enolate of the S-oxo ester moiety of a 2,8-dioxo-6-alkenoate 1 under thermodynamic control at 25 °C using stoichiometric or catalytic amounts of sodium hydride in benzene results in the formation of tram-2-oxo-5-(2-oxoalkyl)-l-cyclopentane-carboxylate 2 exclusively. 

The diastereoselective intramolecular Michael addition of /(-substituted cyclohexcnoncs results in an attractive route to ra-octahydro-6//-indcn-6-ones. The stereogenic center in the -/-position of the enone dictates the face selectivity, whereas the trans selectivity at Cl, C7a is the result of an 6-exo-trig cyclization. c7.v-Octahydro-5//-inden-5-ones are formed as the sole product regardless of which base is used, e.g., potassium carbonate in ethanol or sodium hydride in THF, under thermodynamically controlled conditions139 14°. An application is found in the synthesis of gibberellic acid141. 

An interesting approach to zr n.v-2,3-disubstituted cyeloalkanones is offered by auxiliary controlled intramolecular Michael additions. The diastereoselectivity depends on the chiral alcohol used193> l94. When the borneol derivative 7 was used as substrate, a single diastereomer of 8 resulted when the reaction was performed at 25 "C under thermodynamic control with a catalytic amount of sodium hydride in benzene. 

A ten to hundredfold decrease in the velocity of the reaction, seen as a break down of the Arrhenius plot, is observed at a temperature which, for any given pressure, is always higher than that thermodynamically foreseen for the beginning of the a-/3 transition (this discrepancy is smallest at 265 mm Hg pressure). The marked decrease of the rate of reaction is characteristic of the appearance of the /3-hydride phase. The kinetics of reaction on the hydride follows the Arrhenius law but with different values of its parameters than in the case of the a-phase. 

The hydride phase may be present in a catalyst as a result of the method of its preparation (e.g. hydrogen pretreatment), or it may be formed during the course of a given reaction, when a metal catalyst is absorbing hydrogen (substrate—e.g. in H atom recombination product—e.g. in HCOOH decomposition). The spontaneous in situ transformation of a metal catalyst (at least in its superficial layer) into a hydride phase is to be expected particularly when the thermodynamic conditions are favorable. 

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). 

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