Valence change

Chemists were quick to appreciate Bohr s model because it provided an extremely clear and simple interpretation of chemistry. It explained the reason behind Mendeleev s table, that the position of each element in the table is nothing other than the number of electrons in the atom of the element, which, of course, represents an equal number of periodic changes in the nucleus. Each subsequent atom has one more electron, and the periodic valence changes reflect the successive filling of the orbital. Bohr s model also provided a simple basis for the electronic theory of valence. 

However, the above conclusions are not so simple when the redox couples require different numbers of electrons for reduction, especially if, in addition to the ions whose valence changes, other ions such as H+ are involved. 

D. Quantitative Studies on the Rapid Signal and Molybdenum Valency Changes... 

The limitation of availability of coordination sites in the allyl compounds is further illustrated by the fact that Zr(allyl3)Br dimer reacts with only one molecule of pyridine (without a valency change) to form the monomeric... 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

It should be noted here again that the catalytic reaction does not involve a change of valence of the metal. In general, catalytic olefin addition reactions that involve a hydride transfer do not require change of valence in the metal catalyst. On the other hand, carbon-carbon bond formation by coupling reactions which involve electron shifts, such as in Wilke s Ni°-catalyzed butadiene oligomerization reaction [Eq. (1)], requires a valence change on the metal. 

In a bimolecular reaction, at least two bonds change simultaneously. Figure 22.4 shows how bond orders and valences change in the reaction of methane with a H atom (shown in the deuterium-labeled form that can be studied experimentally)... 

Figure 5.8. Lanthanide Ln203 oxides (cubic cI80-Mn2O3 type, on the left side) and Pb alloys (LnPb3, cubic cP4-type, on the right). The trends of the lattice parameter and of the heat of formation are shown (see the text and notice the characteristic behaviour of Eu and Yb). A schematic representation of the energy difference between the divalent and trivalent states of an ytterbium compound is shown. Apromff represents the promotion energy from di- to trivalent Yb metal, A,//11, and Ar/Ynl are the formation enthalpies of a compound in the two cases in which there is no valence change on passing from the metal to the compound the same valence state (II or III) is maintained.

Cytochrome Any electron transfer hemoprotein having a mode of action in which the transfer of a single electron is effected by a reversible valence change of the central iron atom... 

The nickel-based systems include the flowing systems nickel—iron (Ni/Fe), nickel—cadmium (NiCd), nickel—metal hydrides (NiMH), nickel—hydrogen (Ni/ H2), and nickel—zinc (Ni/Zn). All nickel systems are based on the use of a nickel oxide active material (undergoing one valence change from charge to discharge or vice versa). The electrodes can be pocket type, sintered type, fibrous type, foam type, pasted type, or plastic roll-bonded type. All systems use an alkaline electrolyte, KOH. 

In a similar manner may proceed reactions in which the number of single bonds is increased, as a result not of the breaking of a double bond, but of a valency change of one of the atoms. Thus, for example, the mechanism of the reaction... 

Recent studies have involved some fluorinated monothio-jS-diketones with zinc and nickel (195) and palladium and platinum (196). Both nickel and zinc show valence change of the metal. 

Nevertheless, the inspection of other transition metal series shows that, just as atomic volumes, there are regular variations of cohesive energies when the metal valence changes. Thus, a general increase of about 45 Kcal/mol is found when a metal transforms from a trivalent to a tetravalent state. 

A valence change at the surface may occur as is observed for example for Sm metal ... 

Like other peroxides, also dioxetanes are sensitive to the presence of metal ions and their complexes, which catalyze the decomposition of the dioxetane molecule. In most cases, this decomposition is dark, i.e. no chemiluminesce is generated in such a catalytic cleavage42. An informative exception, for instance, constitutes the chemiluminescent decomposition of the dioxetane 19 in Scheme 13, initiated by the ruthenium complex Ru(bipy)3Cl243. It has been shown that this chemiexcitation derives from the valence change of the ruthenium ion in the process Ru3+ I e — Ru2+, for which the efficiency of the excited-state generation may be as much as 40%44. Hence, when the radical anion of the carbonyl cleavage fragment from the dioxetane and the Ru3+ ion are formed in... 

Since aluminum ion is a strong catalyst, no valence change of the metal ion is involved in the reaction. The following observations of the decarboxylations catalyzed by ferric ion are strongly indicative of the above process. 

Ce3+ (larger cation 0.114 nm). The stress energy caused by this volume change would suppress further valence change of Ce. The substitution of the smaller... 

In the double decompositions just considered, the valencies of the reacting atoms remained unchanged. This is not always so, and a reaction can lead to the formation of an unstable compound which decomposes with a valency change to a lower value. If KI is added to CuCI2, the reaction is... 

The allylic oxidation of propene is catalyzed by (compound) metal oxides, which essentially contain metal ions of variable valency. It is commonly accepted that a redox mechanism is operative in such a way that the catalyst acts as the oxidizer and that lattice oxygen is incorporated in the oxidation products. The assumptions have been proved for several catalysts by the analysis of cation valency changes and by experiments with labelled oxygen. 

Note that it is not necessary to have a balanced chemical equation to work this problem. You need know only the valence changes that are involved. 

Note that, in a problem like this, you need not know the equation for the reaction or the valence changes involved with the reactants. If you wanted to know the molarities of the two solutions, you would have to l w tta has 6 equiv/mole (because CrjQusgoes to 2Cr3+) ar d at Fe2+... 

Notice that whereas in Eq. 16-43 the methyl group is transferred as CH3+ by nucleophilic displacement on a carbon atom, the transfer to Hg2+ in Eq. 16-44 is that of a carbanion, CH3, with no valence change occurring in the cobalt. However, it is also possible that transfer occurs as a methyl radical.420 Methyl corrinoids are able to undergo this type of reaction nonenzymatically, and the ability to transfer a methyl anion is a property of methyl corrinoids not shared by other transmethy-... 

Sm, Eu and Gd can be concentrated by crystallization through double magnesium nitrates, followed by crystallization of bismuth magnesium nitrates [30]. Sm and Eu are then removed by a proceedure based on valence change, and Gd is recovered by bromate crystallizations. Yttrium group earths may be conveniently separated by bromate crystallization. 

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