Formation of Cations

We ve already learned that there are energy changes in the formation of ions, as represented by ionization energies and electron affinities. We also know how those energy changes vary as we move through the periodic table. Those trends correlate very well with the location of metals and nonmetals in the table, and this correlation explains the observations that metals form cations and nonmetals form anions. In each case, the ions we find in compounds are those whose formation is not too costly in energy. 

Using data from Table 6.4 (page 236), predict the ions that magnesium and aluminum are most likely to form. 

Scan the successive ionization energies for each element to find a point where removing one additional electron causes a dramatic increase in the value. It is not likely that sufficient energy would be available to compensate for such a large increase in ionization energy, so the ion formed will be dictated by the number of electrons lost before that jump in ionization energy. 

For magnesium, the first significant jump occurs between the second ionization energy (1451 kj/mol) and the third (7733 kj/mol). We would expect two electrons to be removed, and the most commonly found ion should be Mg . For aluminum, the third ionization energy is 2745 kJ/mol, whereas the fourth is 11,578 kj/mol. We would expect three ionizations to occur, so aluminum should form Al +. 

This type of question is designed to help you see the reasoning behind some of the facts you may already know. In many cases, the way we learn chemistry is by learning information as facts first and then finding out the reason those facts are evident. In the examples from this problem, the answers are consistent with the idea that the ion formed is dictated by the number of electrons lost before the jump in ionization energy. They are also consistent with facts you may already have known, for example, that magnesium will form a 2+ ion because it is an alkaline earth element in Group 2 of the periodic table. 

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Consider first the formation of cations by electron loss. Here the important energy quantity is the ionisation energy. As we have seen (p. 15). the first ionisation energy is the energy required to remove an electron from an atom, i.e. the energy for the process... 

Finally we notice that in the p-type oxides CU2O and NiO, the presence of excess oxygen actually provides, through the formation of cation vacancies, a transport mechanism for the metal, while in an /i-type oxide like TiOi, the excess metal, by forming anion vacancies, provides a transport mechanism for oxygen. With /i-type oxides like ZnO and AljO, where the excess metal is accommodated interstitially, a transport mechanism is, of course, provided for the excess component itself. 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

Macdonald et al.25 28 maintained that the adsorption of chloride ions enhances the formation of cation vacancies of metal ions and their transfer... 

Table 14. Observed (AH obs) and calculated (AH ) heats of formation of cations R + in the gas phase and comparison of heats of formation with enthalpy differences AA (AA = AH°(R+) — AH°(R—F)) in the gas phase and in solution (CH2C12) (all values in kJ mol-1)...

In contrast, the reaction of 147 with 1, in the absence of catalyst, affords traces of adduct after 3 days. The activation by I2 is due to the formation of cationic iodolactonization intermediate 148 (Scheme 4.28) which reacts easily with the diene, affording the dihydrooxazole 149 which is then treated with Bu N to give the final adduct. With some substrates, this method of activation was proved to be more effective than the use of Lewis acids. 

Chlorination of the Cp Ru(amidinate) complexes is readily achieved by treatment with CHCI3, while oxidative addition of allylic halides results in formation of cationic Ti-allyl ruthenium(IV) species (Scheme 243). °... 

This color transformation has been observed in dibenzo-p-dioxin (Structure I) and in its bromo, chloro, nitro, methyl, and ethyl derivatives in addition, the observed electron spin resonance (ESR) signals indicated the presence of paramagnetic species (2, 3). This phenomenon has been attributed to the formation of cation radicals in acid solution. 

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

In the same year, Fulda and Tieke [75] reported on Langmuir films of monodisperse, 0.5-pm spherical polymer particles with hydrophobic polystyrene cores and hydrophilic shells containing polyacrylic acid or polyacrylamide. Measurement of ir-A curves and scanning electron microscopy (SEM) were used to determine the structure of the monolayers. In subsequent work, Fulda et al. [76] studied a variety of particles with different hydrophilic shells for their ability to form Langmuir films. Fulda and Tieke [77] investigated the influence of subphase conditions (pH, ionic strength) on monolayer formation of cationic and anionic particles as well as the structure of films made from bidisperse mixtures of anionic latex particles. 

Scheme 12. Reversible formation of cationic metal-acetylide dendrimers 39 and 40...

Figure 5. Formation of cationic constrained geometry catalysts.

R = II, Me), the reaction with PPh3 leads to the formation of cationic compounds [Au (C-N)Cl(PPh3)]+.1749... 

The time-of-flight secondary ion mass spectrum of a thick film prepared from Si(OEt)4 on a hydrophilic silicon substrate (Fig. 1) reveals a distribution of masses up to 1200 amu. The observed formation of cationized oligomers with a distribution shown in Fig. 1 can be explained by bond cleavage within the uppermost monolayer of the polycondensate of TEOS as a result of primary Ar+ ion impact. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

One of numerous examples of LOX-catalyzed cooxidation reactions is the oxidation and demethylation of amino derivatives of aromatic compounds. Oxidation of such compounds as 4-aminobiphenyl, a component of tobacco smoke, phenothiazine tranquillizers, and others is supposed to be the origin of their damaging effects including reproductive toxicity. Thus, LOX-catalyzed cooxidation of phenothiazine derivatives with hydrogen peroxide resulted in the formation of cation radicals [40]. Soybean LOX and human term placenta LOX catalyzed the free radical-mediated cooxidation of 4-aminobiphenyl to toxic intermediates [41]. It has been suggested that demethylation of aminopyrine by soybean LOX is mediated by the cation radicals and neutral radicals [42]. Similarly, soybean and human term placenta LOXs catalyzed N-demethylation of phenothiazines [43] and derivatives of A,A-dimethylaniline [44] and the formation of glutathione conjugate from ethacrynic acid and p-aminophenol [45,46],... 

Bimetallic activation of acetyl and alkoxyacetyl ligands — through formation of cationic P2 acyl complexes — to reaction with nucleophilic hydride donors was established. Cationic transition metal compounds possessing an accessible coordination site bind a neutral T -acyl ligand on another complex as a cationic P2 acyl system. These i2 3icyl systems activate the acyl ligand to reduction analogous to carbocation activation. Several examples of i2-acyl complexation have been reported previously. 

Metal-excess oxides can change composition by way of metal interstitials or oxygen vacancies. The formation of cation interstitials in a nonstoichiometric oxide MO can be represented by... 

Cp CpZrMe2]/Al203 (5oo) (216 h-1) [185]. Typically, highly dehydroxylated alumina (Al203.(iooo)) is a better support, and it has been associated with the formation of cationic surface species. 

Give both general and specific examples of how organic compounds can lead to the formation of cation exchange sites in soil. 

In addition to this there are some additional peaks which is unusual in the typical organic compounds, The mass spectrum of methane shows m/e values of 14, 13, 12, 2 and 1. This is explained as due to the formation of cationic fragments as follows ... 

Ionic bonding involves the transfer of electrons from one atom to another. The more electronegative element gains electrons. The less electronegative element loses electrons. This results in the formation of cations and anions. Usually, an ionic bond forms between a metal and a nonmetal. The metal loses electrons to form a cation. The nonmetal gains electrons to become an anion. The attraction of the opposite charges forms an ionic solid. 

Surface complex formation of cations by hydrous oxides involves the coordination of the. metal ions with the oxygen donor atoms and the release of protons from the surface, e.g.,... 

Protocols for the estimation of heat of formation of cations have been developed25-28. It appears that heat of formation of positive ions in homologous series is well represented by the equation ... 

The decomposing ionization will take place preferentially by way ofthe electron-hole pair formation, if the formation energy of the electron-hole pair, e, is smaller than the formation energy of the cation-emion vacancy pair, Hv(ab>, and vice versa. In general, compound semiconductors, in which the band gap is small (e,< Jfv(AB>), will prefer the formation of electron-hole pairs whereas, compound insulators such as sodium chloride, in which the band gap is great (e(>Hv(AB>), will prefer the formation of cation-anion vacancy pairs [Fumi-Tosi, 1964]. 

In the case in which the formation of cation-anion vacancy pairs is preferential, the ion levels of A and B ions in solid compound AB are obtained in the same way as Eqns. 3-21 and 3-22 by Eqns. 3-23 and 3-24, respectively ... 

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