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Effect symbiotic

Co3+ when bonded to five CN ligands that are soft, has become soft as a result of the symbiotic effect. Ligands with the ability to bond by using different donor atoms are known as ambidentate ligands. [Pg.583]

Slattery, J. F. and Coventry, D. R. 1995. Acid-tolerance and symbiotic effectiveness of Rhizobium leguminosarum bv. trifolii isolated from subterranean clover growing in permanent pastures. Soil Biology and Biocheistry, 27 111-115. [Pg.283]

In Scheme 1-71, the reversible reaction is shifted to the right when the anion, X, is larger and the cation, M+, is smaller. For example, this shift to the right is 100% in the presence of Na+, PI v, and only 35% in the presence of Na+, F (Hamon Astruc 1988). The equilibrium takes place as an exchange reaction between the two ion pairs. Reactions of this type are based on the symbiotic-effect premise The interaction between a hard cation and a hard anion or between two soft ions is stronger than that between two ions of different types. [Pg.59]

A modification of the HSAB approach was first explained by C. K. Jprgensen. We will consider an actual example to make this idea clear. The Co3+ ion is a hard Lewis acid. However, when Co3+ is bonded to five cyanide ions, a more stable complex results when the sixth group is iodide than when it is fluoride. On other words, [Co(CN)5I]3 is stable, whereas [Co(CN)5F]3 is not. At first this seems like a contradiction that the soft I- bonds more strongly to the hard acid, Co3+. However, the five CN- ions have made the Co3+ in the complex much softer than an isolated Co3+ ion. Thus, when five CFT ions are attached, the cobalt ion behaves as a soft acid. This effect is known as the symbiotic effect, and it indicates that whether a species appears to be hard or soft depends on the other groups attached and their character. [Pg.132]

In protein-stabihzed foams, protein flexibility is critical to the molecule functionality in stabilizing interfaces (Hailing 1981 Lemeste et al. 1990). This has important consequences in the development and stability of dairy foams and emulsions, where the heat treatment received by the material can define its foamability and dispersion properties. A symbiotic effect between native and denatured proteins on the emulsifying properties of whey proteins isolate blends has been observed by (Britten et al. [Pg.296]

In an extension of earlier work, Buigada et al. have also reported on the reaction of the cyclic phosphite (66) with dimethylacetylene dicarboxylate (58) in the presence of proton sources such as carboxylic acids, amide N-H bonds in succinimide or phthalimide and amine N-H bonds in p UTole or indole. With carboxylic acids (67) a mixture of the ylid (68) and the cyclic phosphorane (69) was obtained and in some instances (e.g. with 2,4,6- trimethylbenzoic and p-methoxybenzoic acids) the ylid and phosphorane were shown to be in equilibrium. With amides as the proton source, ylids were generally formed although with N-methylbenzamide (PhCONHMe)a signal attributed to (70) was observed at = - 52 p.p.m. which had disappeared by the end of the reaction through rearrangement to (71). With amines (e.g. pyrrole) the products were again a mixture of ylid (72) and phosphorane (73) and the entire set of results was rationalised in terms of HSAB theory and the symbiotic effect around phosphorus. [Pg.55]

Allylic and propargylic 3-keto sulfoxides could be reduced as well as saturated compounds. Optically active allylic 3-hydroxy sulfoxides present some specific interest because of the possible hydroxylation of the double bond leading to vicinal triols. The osmium tetroxide catalyzed hydroxylation reaction of the double bond in the presence of trimethylamine N-oxide is highly stereoselective the (/ ,/ )-3-hydroxy sulfoxide gave only one diastereoisomeric triol as a result of a cis hydroxylation of the double bond and a symbiotic effect of the two chiral centers in the asymmetric induction (the (S,/ )-isomer gave a lower de). [Pg.156]

Also, in Table 1.4, CF is shown as a harder acid than CH. These are examples of a very general phenomenon, first noted by Jorgensen and called by him the symbiotic effect. Soft bases attached to the same central acceptor atom make it a soft acid, and hard bases make it a hard acid. In coordination chemistry, symbiosis explains why some ligands, such as CN or phenanthroline, make a metal ion form strong complexes with other soft ligands, whereas F and H2O favor the bonding of other hard ligands. [Pg.15]

The symbiotic effect is also common in organic chemistry, but here it has been called the clustering, anomeric, or geminal effect.Clustering refers to the stabilization caused by adding several substitutents to the same carbon atom. Some extreme examples are shown by Reactions (1.20) and (1.21), in which the number of bonds of each kind is preserved. [Pg.15]

By comparing AG with AG°, we hope to have accounted for the dramatic effect of the solvent on lowering the reactivity of hard bases. Looking at Reaction (1.25), it appears that the enhanced reactivity of soft bases must be accounted for by favorable interactions between B and B in the TS, B A B. But this is just the symbiotic effect, since the leaving group, B, is iodide ion, which is soft. The existence of this symbiotic effect on rates has been known for some time. ... [Pg.20]

Symbiotic effects are a serious problem in the use of the Marcus equation to predict rates of methyl group transfers." This otherwise useful equation predicts that for a pair of reactions such as... [Pg.21]

The final chapter of this section is by Rappoport and is concerned with nucleophilic reactions at vinylic carbon. Two reaction types are considered, those of neutral vinyl derivatives and those of vinyl cations. Correlation of rates for these reactions with both Ritchie and Swain-Scott equations was attempted without success. Rappoport concludes that these reactions are subject to a complex blend of polar, steric, and symbiotic effects and that a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed . This conclusion is reminiscent of the earlier observation of Pearson (see the introduction to the section on the Brpnsted equation) and the later observation of Ritchie (Chapter 11) regarding the difficulty of correlating nucleophilic reactivity with a single equation. Rappoport finds another familiar situation when he explores the relationship between reactivity and selectivity for the vinyl substrates sometimes the RSP is obeyed and sometimes it is not. [Pg.26]

The nucleophilicities are found to be dependent on electronic, steric, and symbiotic effects, and limited series obeyed a constant selectivity , a reactivity-selectivity or a dual-parameter linear free-energy relationship. The conclusion made was that because of different blends of the effects, the construction of a substrate-independent nucleophilicity scale was impossible at present, but an approximate scale was presented. In nucleophilic reactions on relatively long lived vinyl cations, the steric effects predominate, but at constant steric effects, reactivity-selectivity relationships were found for very short series of substrates. Additional data are required for constructing more reliable nucleophilicity scales toward neutral and positively charged vinylic carbons. [Pg.390]

Table IV compares the reactivity ratios of a soft (PhS-) to a hard (MeO-) nucleophile in vinylic substitution. PhS is always more reactive, and ratios lower than unity, as for 4, X = Br (4), are certainly due to elimination-addition with MeO . The ratios change by >2000-fold and are sensitive to the geometry of the substrate. An important feature is that for (3-halo-p-nitrostyrenes the ratio decreases strongly with the increased hardness of the (3-halogen (38). The lowest ratios are for the (3-fluoro derivative, whereas the differences between the chloro and bromo compounds are not so large. This behavior is similar to that in SNAr reactions. This behavior can be rationalized by symbiotic effects, which favor the soft-soft PhS--Br interaction and the hard-hard MeO-F interaction. A reactivity-selectivity relationship for vinyl bromides of different electrophilicities does not exist. Table IV compares the reactivity ratios of a soft (PhS-) to a hard (MeO-) nucleophile in vinylic substitution. PhS is always more reactive, and ratios lower than unity, as for 4, X = Br (4), are certainly due to elimination-addition with MeO . The ratios change by >2000-fold and are sensitive to the geometry of the substrate. An important feature is that for (3-halo-p-nitrostyrenes the ratio decreases strongly with the increased hardness of the (3-halogen (38). The lowest ratios are for the (3-fluoro derivative, whereas the differences between the chloro and bromo compounds are not so large. This behavior is similar to that in SNAr reactions. This behavior can be rationalized by symbiotic effects, which favor the soft-soft PhS--Br interaction and the hard-hard MeO-F interaction. A reactivity-selectivity relationship for vinyl bromides of different electrophilicities does not exist.
From this discussion, clearly, a quantitative nucleophilicity scale toward vinylic carbon cannot be constructed. Neither Ritchies nor Swain-Scott s correlations are applicable. Different blends of contribution of polar, steric, and symbiotic effect can change the reactivity order. Whether a qualitative order prevails could be inferred by comparing the three substitution reactions of chloro olefins, which are the only processes for which a relatively extensive change in the type of nucleophile was conducted (Table X). [Pg.402]

These results may be understood in the context of soft-hard acid-base theory.As mentioned earlier, the Fischer carbene complexes can be regarded as soft electrophiles, especially the alkylthio complexes. Hence, the adducts 99 formed by the reaction of 98b with a thiolate ion nucleophile enjoy enhanced stability due to the symbiotic effect of adding a soft nucleophile. This stabilization apparently reduces the need for additional stabilization by the phenyl substituent, which translates to a reduced p(Ki) value. [Pg.191]

Smith GJ. (2008). Symbiotic effects of biodegradation during electrical resistance heating. Proceedings of the Sixth International Conference on Remediation of Chlorinated and Recalcitrant Compounds, May 19-22, 2006, Monterey, CA. [Pg.534]

Si(OR)3 is harder than SiHs, accordingly [7]. In Table 2, we see that BFj is hard and BH3 is soft. The explanation for the symbiotic effect is readily seen. In BF3, the bonding is very ionic and boron has an actual charge close to its oxidation state of 3 +. In BH3, the bonding is covalent and boron has an actual charge of close to zero. Increased positive charge on the acceptor atom always increases the hardness. [Pg.6]

The HSAB concept is helpful here, too each soft or hard fiagment strives for stabilization on a corresponding center (symbiotic effect). Complex A exhibits a hard/ soft dissymmetry (NH3 is hard, 1 soft), whereas in complex B the hard Co " center is stabilized exclusively by hard ligands. [Pg.19]

The methylation of A, A/ -bis(trimethylsilyl)hydrazine (79) occurs with partial rearrangement, which may be interpreted by the symbiotic effect. In the anomalous product each nitrogen atom is bonded twice to the same substituent as well as to one another. [Pg.22]

The idea of symbiosis has been extended to 8 2 reactions by Pearson and Songstad (80). Analysis of rate data from the following reactions in methanol yielded a pattern consistent with symbiotic stabilization of the five-coordinated transition state. Later, it was demonstrated that the symbiotic effect is even larger in aprotic solvents such as acetonitrile (81). Gas phase 8 2 reactions also display this effect (82). Leaving group symbiosis is important in determining the alkylation site of acetoacetic esters (83). Apparently transition state symbiosis also operates in aromatic substitution (8 Ar)(84). [Pg.22]


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