Substitutional lability

The superb elegance of this demonstration lies in the choice of reactants which permits no alternative mechani.sm. Cr" (d ) and Co" (d ) species are known to be substitutionally labile whereas Cr" (d ) and Co " (low-spin d ) are substitutionally inert, Only if electron transfer is preceded by the formation of a bridged internrediate can the inert cobalt reactant be persuaded to release a Cl ligand and so allow the quantitative formation of the (then inert) chromium product. Corroboration that electron transfer does not occur by an outer-sphere mechanism followed by los.s of CP from the chromium is provided by the fact that, if Cl is added to the solution, none of it finds its way into the chromium product. 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

The rates of oxidation of V by complexes of the type Co(NH3)sX, where X = H2O, NH3 and Cl , have been examined by Zwickel and Taube in H2O and in D2O solution. These workers have compared the results on such systems with data on the Cr +Co(NH3)6 reaction in H2O and D2O. The product unlike Cr, is substitution-labile. Consequently, a different approach is necessary for oxidations than is customary for Cr oxidations. The specific rate coefficients for Co(NH3)g and Co(NH3)6 " + Cr vary with Cl concentration according to... 

Reactions (6), (7) and (9) occur with substitution-labile oxidants and (6), (7) and (8) with substitution-inert oxidants. It is postulated that the species (MS03)" involves metal-oxygen coordination for labile M", and metal-sulphur coordination for inert M". The latter mode is thought to prevent sulphur-sulphur bond formation. 

Although somewhat more stable than its hexaammine relative, the air-sensitive [Co(en)3]2+ is still substitutionally labile and racemizes rapidly in solution. Chiral discrimination in its (racemic) solutions has been observed in outer sphere electron transfer reactions with optically active oxidants including [Coin(EDTA)], 209,210 [Cr(ox)3]3-,211,212 Co111 oxalate, malonate, and acetylacetonate (acac) complexes.213... 

As Cu11 is substitution-labile,154 the rates of mass transfer are dependent on interfacial processes, which have been shown155 to be fast for both loading and stripping in conventional contactors, but possibly too slow in stripping for the industrial application of columns. 

The development of new technology for extraction and recycling of palladium has been stimulated by its increasing use in automobile catalysts, which now consume ca. 55% of world production. A range of options for extraction from acidic media and for separation from other PGMs is possible because Pd11 is relatively substitution-labile. This allows cationic exchange, e.g.,... 

Some other reactions of metal nitrosyls LxM(NO) with various nucleophiles (Nuc) are summarized in Table III. The pattern indicated by the studies described above is repeated simple adduct formation occurs when the coordinated nitrosyls are sufficiently electrophilic and the nucleophiles sufficiently basic. The first species formed is probably the N-coordinated nucleophile nitrosyl adduct LrM(N(O)Nuc), e.g. Eq. (27). Subsequent reactions depend on the substitution lability of these species, as well as on the redox stability of the complex and of the ligand. 

The outer sphere mechanism may take place in all redox active systems while inner sphere mechanism requires substitutionally labile reactants and products. 

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

The Fe + complex of the tetradentate NaO-donor tripodal ligand (168) provides a model for mononuclear nonheme bioactive sites. [Fe (168)Cl2], whose chloride ligands are substitution-labile, reacts with 2 in DMSO solution to give an Fe .02 species that may well be identical with the stable Fe 02 species obtained from the reaction of the iron(II) complex of (168) with 02. ... 

While nitroprusside itself is substitution inert, the reduction product, [Fe(CN)5(NO)] , rapidly loses the trans cyanide (see above) and [Fe(CN)4(NO)] is substitution labile via a dissociative pathway and is rapidly substituted by chelating diamines to give products such as... 

This section is separated from the former one, even though mixed addenda POMs are mostly, if not exclusively, synthesized from the appropriate lacunary precursors and can be viewed normally as substituted POMs. At least two reasons can be invoked to justify such dichotomy first, a substitutionally labile position is available on the transition metal substituted into the POMs framework, which is not the case with mixed addenda compounds second, these transition metal centers are usually the active sites for catalytic and electrocatalytic reactions and might deserve special attention. 

Substitutionally labile metal complexes often generate hydroperoxides by direct substitution with H202 (86 90) or in the reactions between 02 and the reduced metal (91). These mechanisms are commonly observed in naturally occurring molecules and their mimics in the processes of activation of oxygen and hydrogen peroxide (92-98). 

The Cr—C bond exerts a considerable (ca. 0.1 A) solid-state trans elongation (Table I), and the trans effect is implicated in the enhanced substitution lability of the trans aqua ligand. A variety of chelating systems can be used to form [L6CrR]2+ cations, including diamines (63), polyamines, macrocyclic N4 ligands, acac, bipy, and polycarboxylates such as nta, edta (64), and hedtra. 

The apparent reactivity of the hydride end of the dinuclear hydridomethyl species is in line with the considerable substitutional lability noted above for H2Os2(CO)8. This reactivity contrasts with the comparative inertness (mentioned above) of the mononuclear hydride Os(CO)4H2 and of Os3(CO)i2. Its origin is unclear, but it makes the dinuclear hydrides and hydridoalkyls excellent starting points for the synthesis of more complex polynuclear alkyls. For example,... 

A possible mechanism for manganese (III) catalysis would be the stepwise formation of a manganese (III) complex with two peracetic acid molecules which then would decompose in the rate-determining step. The formation of this complex should be rapid since it is known that manganese (III) complexes are substitution labile (6, 23) ... 

The sequence of Reactions 19, 20, and 21 indicates that substitution on the metal ion by at least one of the substrates is faster than the over-all reaction rate. This can be understood easily for manganese (III) complexes (substitution labile). It is, however, more difficult to rationalize for cobalt (III) unless the rate of cobalt (II)-cobalt (III) exchange is very fast in this system. 

In contrast to the 18-electron systems, the 17-electron V(CO)6 is substitutionally labile. It is one of the few 17-electron systems that can be studied under non-transient conditions but there is considerable evidence that the conclusions drawn from the study of this system can be applied to transient 17-electron species such as Mn(CO)5. Data from a published study of CO exchange with V(CO)6 are absent but there is much anecdotal reporting. For example, a t/2 value of 7 h for the reaction in heptane at 10 °C under 2 atm of CO is quoted.97 The substitution reactions of V(CO)6 are associatively activated (k2 for the reaction between V(CO)6 and PPh3 in hexane at 25 °C =... 

The need for the presence of at least one amine proton in the substrate and a wealth of experimental evidence support a mechanism in which the lyate ion (or even the solvent itself) acts as a base to remove a proton from a suitably placed amine group, thereby generating a substitutionally labile amido species. There is strong evidence to support the idea that the mode of activation of this species (at least in the case of the Co111 species) is dissociative, but there is still disagreement as to whether the stoichiometric mechanism is dissociative (Z)) or interchange (7d). The distinction between these mechanisms rests upon the presence or absence of an identifiable five-coordinate intermediate that has lost all memory of its origins. 

In the absence of pKA data for the individual amine protons in these substitutionally labile complexes it is not possible to evaluate k2 and thereby measure precisely the amount by which deprotonation increases the lability of the complex. Even if such data were available it would be difficult, in complexes containing more than one type of amine proton, to be sure whether the two quantities related to the same proton. In spite of this major drawback it is possible to estimate orders of magnitude. The evidence suggests that in Co111 complexes the amido conjugate base can be between 105 and 1013 times more labile than the amine complex from which it was derived.301 The effect is far less marked in equivalent complexes of Crm, Ru111 and Rh111352-353 and, so far, there... 

---