Stability rules

Using nuclide-stability rules, form a hypothesis that explains why calcium-40 should be a more stable nuclide than potassium-40. 

The well-known rule that positive oxidation states are stabilized by coordination may be called the first stabilizing rule. This rule relates oxidizing properties and complex stability the higher the complex stability, the more negative the redox potential. We can also say that... 

The first stabilizing rule can also be applied to redox systems involving cationic species of different charge in the reduced and the oxidized species, respectively ... 

According to the functional approach, the first stabilization rule is considered to result from the dispersion of positive charges from the coordination center over the whole complex and hence from a decrease in the net positive charge of the redox-active species, which corresponds in extent to the EPD properties of the ligands. The relationship between the standard redox potential and the EPD properties (donicities) of the ligands has been clearly established. 

We may formulate the second stabilizing rule as follows Anions are stabilized by coordination with EPA with a simultaneous change in standard, redox -potentials to positive values this change in redox potential due to coordination by EPA is related to the stability of the complex species. 

The second stabilizing rule suggests that the anion, or more generally speaking, the EPD unit, may be the coordination center in a complex compound2). The EPA unit is consequently considered the ligand. 

We may call the stabilizing rule for back-donation by extremely weak coordination centers and extremely weak hgands by the third stabilizing rule. 

Further reduction to cobalt (I) further increases the electron population of the coordination center and the radical-bonding properties of cobalt are no longer favored. Instead, the EPD properties that prevail at the coordination center allow coordination by EPA units according to the second stabilizing rule the complex ion is stabilized ) as a hydrido complex ... 

Since Cu(I) is considerably softer than Cu(II), back-donation is further increased (as expected according to the third stabilizing rule) and chlorine is finally replaced by a thiourea molecule ... 

Silicon tetrafluoride is tetrahedral but SF4 has a see-saw or butterfly shape. Carry out the VSEPR predictions of these shapes. Consider the MO treatment of SiF4 as a o-only case, by using Figure 6.3. Decide if the distortion of a tetrahedral SF4 to the C,v symmetry is consistent with the HOMO/LUMO stabilization rule, discussed in Chapters 5 and 6. 

These reactions are two examples of a process used for obtaining metals from oxides, in which the metal is said to be reduced9. Since technical processes are concerned only with the production of the metal they are called reduction processes, overlooking the fact that hydrogen is oxidized during the reaction. Actually, these reactions consist of the substitution of one positive ion in an oxide by another. From the stability rules it is possible to predict which metal will... 

The compounds of elements of the first three columns of the periodic system are not reduced by hydrogen, and those in the subsequent columns only partly so. At high temperatures TiCl4 is easily reduced to TiCl3, but the reaction proceeds no farther because the charge of the titanum ion is reduced. Further complications can occur in the reaction of a metal with a compound if the two metals can, themselves, combine to give a compound. In such instances the reaction will proceed in quite a different way from that expected from the stability rules. Because of the different heats of formation of halides and chalcides it can happen that a given element, for example Na, can reduce a chloride but not an oxide. 

The stability rules can also provide information as to what will happen when two compounds react together. As a first example we will take the type of reaction in which both reacting compounds consist of ions, all of which have the same charge. 

The stability rule still holds even when the complex ion is no longer purely ionic in character, so that we cannot use its validity as an argument that CO and SOf are built up from ions, because the rule holds for complexes to which this does not apply. The most it does show is that the bond between the complex ion and the rest of the compound is ionic in character. 

Since a complex like KMgF3 does not contain isolated complex ions, the reasoning by which the stability rules for complexes were derived in the preceding sections does not hold for complexes not containing isolated complex ions. In this section it will be shown that, for the latter group, the stability rules are valid, too. 

Normally, it is not possible to obtain analytical solutions for this transport problem and so we cannot a priori calculate the reaction path. Kirkaldy [J. S. Kirkaldy, D. J. Young (1985)] did pioneering work on metal systems, based on investigations by C. Wagner and the later work of Mullins and Sekerka. They used the diffusion path concept to formulate a number of stability rules. These rules can explain the facts and are predictive within certain limits if applied properly. One of Kirkaldy s results is this. The moving interface in a ternary system is morphologically stable if... 

In this chapter, recent developments in the regioselective, site-selective, and stereoselective preparation of oxetanes have been summarized. The relative nudeophilicity of the alkene carbons was seen to be important for regioselectivity, in addition to the well-known radical stability rule. Likewise, the three-dimensional structures of the triplet 1,4-biradicals were seen to play an important role in stereoselectivity. For photochemical reactions that proceed via radical ion pairs, the spin and charge distributions are crucial determinants of regioselectivity. It follows that the concepts used in selective oxetane synthesis should stimulate future investigations into the mechanistically and synthetically fascinating Paterno-Bitchi-type reactions. 

Figure 5 Application of the topological charge stabilization rule to bicyclic heterocycles.

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