Natural order of stability

The natural order of stabilities in operation consistent variation of measured stability constant with different chelate ligands for the series of transition metal(II) ions from manganese across to zinc. 

The stabilities of high-spin complexes of the ions between Mn " and Zn with a given ligand frequently vary in the order Mn < Fe < Co < Ni < Cu > Zn. This order, sometimes ealled the natural order of stability, is relatively consistent with the charge-to-radius concept, sinee the radii of the ions vary in the order Mn > Fe > Co > Ni < Cu < Zn. Both the variation in size of the cation and the order of stability ean be explained in terms of the crystal field stabilization energy (CFSE) for these complexes (Section 2.5). 

Figure 5.2 indicates the relative stability of high-spin oetahedral [M(n)L6] complexes of first-row transition elements as predicted by CFT. The (f and (f systems will be the most stable with respeet to their neighbors, since they have the greatest CFSE. As one progresses from Ca to Zn eomplexes, a general increase in stability is observed. The order of stability predieted by CFT, and presented in Figure 5.2, parallels the natural order of stability for complexes of... 

Dealkylation is the main direction in the decomposition of mixed dications 124-126. Stability of these dications strongly depends on the nature of the chalcogen and substituents.119 In general, the order of stability is consistent with the difference in electronegativities and changes as follows Te > > Se > S (Scheme 46).95 119... 

Irving H, Williams RJP (1948) Order of stability of metal complexes. Nature 162 746-747... 

Several major trends are apparent in Table XXV. (1) The redox potentials for any given ligand vary greatly with the nature of the metal. This correlation argues strongly that the electrochemical processes are primarily metal centered. An outstanding result is the exceptional stability toward oxidation of the Cr(III) d3, and Co(III) d6 complexes. (2) The values depend on the substituent R and show consistently that the order of stability of the oxidized or reduced species varies from the dibenzyl complexes to the dicyclohexyl complexes. The former are easier to reduce and more difficult to oxidize, while the latter are the easier to oxidize and more difficult to reduce. 

The importance of complex formation is exemplified by the binding of Mg, Na, Co, Mn, Fe, Cu, and Zn to fulvic acids (FA), and of Fe to humic acids. The binding capacity of these natural acids for metal ions is within the range of 0.2-0.6 mmol/g, and the order of stability of complex formation (M-FA) with some key metals is Fe2+ > Al3+ > Cu2+ > Ni2+ > Ca2+ > Zn2+ > Mn2+ > Mg2+(see Schnitzer, 1970). Interestingly, some cryptogams (i.e., mosses and lichens) capture part of their essential minerals by secreting... 

The thermal stability of polyindanes obtained from 59a-d varied with the nature of the group X. The order of stability for the four X groups was O > CH2 > CH2CH2 > S. The polymer obtained from the polymerization of 67 had stability equivalent to that obtained from 59c. 

The stability of the radicals depends on the nature of the atom that is the radical centre and on the electronic properties of the groups attached to the radical. As in the case of carbocations, the order of stability of the free radicals is tertiary > secondary > primary > methyl. This can be explained on the basis of hyperconjugation as in the case of carbocations. The stability of the free radicals also increases by resonance possibilities. Thus, benzylic and allylic free radicals are more stable and less reactive than the simple alkyl radicals. This is due to the delocalization of the unpaired electron over the Tr-orbital system in each case. 

The order of stability also depends on the nature of the central atom (Si > Zr, Ti > Ge > P > As) and on the nature of the surrounding anion groups (W > Mo > V). This classification has some relevance to hydrotreating catalysts if, for example, introduction of Si into alumina as a support (giving silica-alumina, zeolite, etc.) leads to the formation of Mo —Si or W —Si heteropoly compounds, they will not be degraded during calcination. 

A number of empirical rules have been proposed to deduce the relative order of stability of polymorphs and the nature of the process that interconverts these (i.e., enantiotropy vs. monotropy). Among the better known are the Heat of Transition Rule, which states that if an endothermic transition is observed at some temperature, it may be assumed that there must be a transition point located at a lower temperature where the two forms bear an enantiotropic relationship. Conversely, if an exothermic transition is noted at some temperature, it may be assumed that there is no transition point located at a lower temperature. This in turn implies that either the two forms bear a monotropic relationship to each other or that the transition temperature is higher than the temperature of the exotherm. 

In deep-fat frying, the temperature reaches >170°C, and tocopherols are unstable. The order of stability of different tocopherols remains similar to that at lower temperatures, i.e., a < y < 8 (Gordon and Kourimska, 1995 Lampi and Kamal-Eldin, 1998). However, an opposite order of stability has also been found. During deep-fat frying, the relative stabilities of natural tocopherols and tocotrienols in soybean oil were a-tocopherol > 8-tocopherol > P-tocopherol > y-tocopherol, in corn oil were a-tocopherol > y-tocopherol > 8-tocopherol > y-tocotrienol, and in palm oil were a-tocopherol > S-tocotrienol > a-tocotrienol > y-tocotrienol, respectively (Simonne and Eitenmiller, 1998). The authors assumed that tocotrienols were less stable than tocopherols, because they acted more effectively as antioxidants. 

Tautomeric equilibria of uracil, thymine, and 5-fluorouracil in solution are sensitive to the solvent polarity and, in contrast to the gas-phase equilibria, to the nature of the 5-substituent. In all the cases, however, the dioxo tautomer 158a is still the most stable. In low polarity solvents, such as dioxane, the order of stability is similar to that in the gas phase and unaffected by the 5-substituent (01MI103). In more polar acetonitrile and water, the tautomer 158c (for uracil and thymine) or 158e (for fluorouracil) is better stabilized by solvation than other tautomers, as confirmed by the reaction field... 

A study of the stability of four natural sedimentary zeolites indicated the order of stability in an acidic environment was found to be Trinity Basin, NV mordenite > Hector, CA clinoptilo-lite > Eastgate, NV erionite > Bowie, AZ chabazite. 

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