Ion Stability

On a strictly statistical basis, one would expect that the relative number of H2O and NH3 molecules surrounding Cu would be the same as the relative 

In some cases, the preferences of metals for certain ligands is easily understood it seems reasonable that positive metal ions should prefer anionic ligands to neutral or to positively charged ones. In general, however, the factors that determine which ligand will coordinate best with a given metal ion are numerous, complicated and not completely understood. Some of these factors will be discussed later in this chapter. 

The properties of a metal ion in solution are dependent on the nature and number of the groups (ligands) surrounding the metal. The number and type of such groups cannot be predicted easily. Consequently, many studies have been made to establish the composition of the coordination spheres of metal ions in solution in the presence of a wide variety of possible ligands. In discussions of the solution behavior of coordination compounds, it is normally assumed that the solvent is water, but nonaqueous solvents will dissolve certain 

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Both experimental studies on gas-phase ion stability and MO calculations indicate that the two vinyl cations shown below benefit fiom special stabilization. Indicate what stmctural features present in these cations can provide this stabilization. 

An active-site zinc ion stabilizes negative charge development on the oxygen atom of acetaldehyde, leading to an induced partial positive charge on the carbonyl C atom. Transfer of the negatively charged hydride ion to this carbon forms ethanol. 

It has been possible, as already noted (footnote, p. 1240) to study the spectra of Ln ions stabilized in CaF2 crystals. It might be expected that these spectra would resemble those of the -)-3 ions of the next element in the series. However, because of the lower ionic charge of the Ln" ions their 4f orbitals have not been stabilized relative to the 5d to the same extent as those of the Ln " ions. Ln" spectra therefore consist of rather broad, orbitally allowed, 4f 5d bands overlaid with weaker and much sharper f f bands. 

Radical stability can often be explained in the same way as ion stability molecules that delocalize unpaired electrons tend to be more stable. Display spin density surfaces for 1-propyl and 2-propyl radicals. In which is the unpaired electron more delocalized Is this also the lower-energy radical ... 

A meehanistie seheme that was proposed involves the sueeessive or synehronous elimination of a proton and a ehloride ion from the 5-methyl group and from the ehloroethynyl group attaehed to it, followed by addition of the nueleophile (NH3) to the intermediate bipolar earbene ion stabilized by eonjugation (Seheme 103). 

Most of the work on pyroaurite materials has been done on materials with Fe [68-72, 76, 77], Co [68, 75, 78], Mn [72, 79], or Al [68, 70, 72] substitutions. When at least 20% on the Ni atoms are replaced by the trivalent substituent, the materials are stable in concentrated KOH. In many ways the pyroaurite phase is similar to -Ni(OH)2. Thus substitution of 20% of the Ni with these trivalent ions stabilize the operation of the electrode in the al y cycle in concentrated KOH. 

The following conclusions emerge. The rate constant of the monomer addition to oc-lithio-iso-butyrate (the monomeric living polymethyl methacrylate ) is much greater than any other, the ratio kt / k2 20 reflecting the ion stabilizing effect of the... 

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

Substituents Relative Arenium Ion stability Relative n-Complex Stability Rate of Chlorination" Rate of Nitration ... 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Solvent and Simple Ion-Stabilized Metal Nanoclusters Chemical Synthesis and Application... 

Of all the known sites for metal-ion binding to the heteroatoms of DNA bases, G-N3 is the most elusive. The adjacent 2-amino group is often considered to offer steric hindrance to binding at this site. However, while this undoubtedly influences the chemistry it does not preclude binding. The tri-metalated [ [Pt(N]3(9-Et G N1,N3,N7)]5 compound has for many years been the only structurally characterized example of an N3-coordinated guanine (66). A second example has now been reported, the tetranuclear octacation 16 (56). In this complex both the N7 and N3 atoms are bound to Pd2+ (Fig. 22). The molecule presents an interesting new architecture for a guanine-tetramer. Such structures are well known in DNA chemistry and are almost inevitably metal-ion stabilized (67,68). 

Compartmental pyrazole ligands (14) can be used to form bimetallic complexes (15).157 This ligand enforces a separation distance between the metal ions. Stability constants were calculated showing a stability for both mononuclear and dinuclear compounds that was less than Cu11 but greater than Nin. 

Veley, C.D. "How Hydrolyzable Metal Ions Stabilize Clays to Prevent Permeability Reduction" SPE paper 2188, 1968 SPE Annual Meeting of AIME, Houston, September 29-October 2. 

FIGURE 5.1 Major fragmentation pattern of the parent ion in propane. Numbers on the curves denote m/e values. The relative abundance of these, as well as of the minor ions, stabilizes beyond -40 eV. Adapted from Eyring and Wahrhaftig (1961), with permission from Am. Inst. Phys. ... 

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