Stoichiometry Stress, metals

Some measured values of hardness are given in Table 8.1 which shows how the hardness varies with stoichiometry (Qian and Chou, 1989). The values in the table are averages of 30 measurements for each composition. The stoichiometric value is 16X the yield stress (albeit from different authors). Since hardness numbers for metals are determined by deformation-hardening rates, the latter is very large for Ni3Al causing the hardness numbers to be 16X the compressive yield stress instead of the 3X of pure metals. 

It should be stressed that the coding for the formation of these topologically complex molecules needs to be carefully controlled in order to obtain the desired structures. To illustrate this, consider ligand 7.59, which contains two didentate metal-binding domains. This might be expected to react with octahedral metal ions to give a triple-helical dinuclear complex. Reaction with iron(n) does indeed give a species of stoichiometry [Fe2(7.59)3]4+ however, the crystal structure reveals that an untwisted complex, 7.60, has been formed. 

It must be stressed that factors such as the hydration (or solvation) of the metal ion and anion effects on the extracted complex often make it difficult to predict the order of extractability for such systems. Such factors may even influence the stoichiometry of the extracted species. Thus, the simple match of the metal to the whole concept is only of limited utility. For example, potassium, rubidium and sodium nitrates are extracted in the presence of dibenzo-18-crown-6 (2) as 1 1 1 complexes. On the other hand, cesium forms a 1 2 1 sandwich complex with this crown (metal crown nitrate) in the organic phase and this affects the extraction order for the above metal ions, with the order being dependent on ligand concentration. In contrast, for picrate as the anion the composition of the extracted cesium complex is 1 1 1 (Fig. 4.8) [27]. 

Voorhoeve et al. (14,30) have also stressed that the catalytic activity of perovskites is influenced by their stoichiometry. A simple way of varying the oxidation state of the ion at the position B is by substitution of the A ion by a different ion with an oxidation state other than 3. This method has been used by several authors (9, 62, 88, 96, 179-181) to understand the role of the 3orbital occupancy in the LaM03 series on the catalytic oxidation of CO. For M = Co the appearance of Co2+ ions by introduction of Ce4+ in position A enhances the rate of oxidation of CO, whereas the presence of Co4+ ions by substitution with Sr2+ reduces the rate. The explanation for this behavior has been given by assuming that CO is bonded to the transition-metal ion as a carbonyl, as occurs on metals (182), with donation of the carbon lone pair into the empty 3dzi orbital of M to form a cr-bond accompanied by back-donation of the f2g electrons of the metal to the antibonding rr-orbital of CO. It should be noted that the dz2 orbital is the lowest et level for the M3+ ions at the surface, and in order to have a partially empty dzi level, the occupation of all the et levels must be below unity. 

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