Bond rupture mechanism

A rate and equilibrium study of the cis/trans isomerization of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) in the vapour phase has been reported.761 The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 °C, the same value being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-1 the Cr—O bond energy is 210 kJ mol-1. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. 

The classical experiment tracks the off-gas composition as a function of temperature at fixed residence time and oxidant level. Treating feed disappearance as first order, the pre-exponential factor (kr and activation eneigy, E, in the Arrhenius expression (eq. 35) can be obtained. These studies tend to confirm large activation energies typical of the bond rupture mechanism assumed earlier. However, an accelerating effect of the oxidant is also evident in some results, so that the thermal rupture mechanism probably overestimates the time requirement by as much as several orders of magnitude (39). Measurements at several levels of oxidant concentration are useful for determining how important it is to maintain spatial uniformity of oxidant concentration in the incinerator. 

This type of isomerization cannot take place by a twisting process and requires a metal-ligand bond rupture mechanism. 

One last example, the racemization of a tris (bidentate) ion by an intramolecular mechanism in which one end is temporarily uncoordinated is shown in Figure 6, where A = G. The bond rupture mechanism is considered the probable mechanism for racemization of unsymmetrical /3-diketone complexes based on the nuclear magnetic resonance studies by Fay and Piper 39). Extension of the scheme shown in Figures 1 and 2 to other systems is almost limitless. 

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

The quantum yield of photoracemization of [Cr(en)3] has been foimd to be independent of pH. Although some of the data obtained suggest that the twist mechanism of photoracemization might be preferred, it has not proved possible to discriminate definitively between this and the bond-rupture mechanism. In aqueous HCl solutions, irradiation of [Cr(en)3] or [Cr(NH2Me)6] is reported to lead to substitution of one ligand by water. Photoaquation also occurs in... 

Studies of the kinetics and steric course of the first-order intramolecular stereoisomerizations of the Al /3-diketonates tris(2,6-dimethylheptane-3,5-dionato(aluminium(iii) and bis( 1,1,1,5,5,5 -hexafluoropentane-2,4-dionato)alu-minium(m) have been made. It appears that chelates having alkyl or aryl substituents on the /3-diketonate rings isomerize by a rhombic twist mechanism. If fluorocarbon substituents are present, however, a bond-rupture mechanism is more likely, with formation of a square-pyramidal apical five-co-ordinate inter-mediate. ... 

Rate data from H n.m.r. investigations of the intramolecular rearrangement processes involving a number of jff-diketonate complexes of V are collected in Table 4. An unambiguous distinction between twist and bond-rupture mechanisms could not be established. The V complexes rearrange significantly faster than the analogous Al species and at rates comparable to those of Mn and Ga. ... 

Miscellaneous.—twisting rather than a bond-rupture mechanism has been established by n.m.r. line-broadening studies for the racemization of Ti complexes of the type [Ti(AA)(BB)2] [BB = acac or 3-acetylpentane-2,4-dionate ion AA = MeaC(0-)(CH2) C(0-)Me2, n=0 or 1]. ... 

Enthalpies of activation have been determined for the dehydration, dehydrohalogenation, and isomerization of cis- or trans-[Cr(LL)2X2]X H20 (X = Cl or Br LL = (f,/-2,3-butanediamine, 1,2-cyclo-hexanediamine, and 2,4-pentanediamine). A bond-rupture mechanism is proposed for the isomerization, the opposite conclusion to an earlier proposal that a twist mechanism is involved. " ... 

The quantum yield for the photoracemization of A- and A-[Cr(en)3] " ion in acid solution is 0.015. Although the data do not rule out a bond rupture mechanism, the twist mechanism is favored. 

The authors conclude that for the corresponding substitution which is not acid catalysed, the mechanism cannot involve the rupture of an aluminium-oxy n bond in the rate-determining stage, as this would be too slow a process to account for the observed rate. Other authors have concluded - that the stereoisomerization of the aluminium(iii) / -diketonate complexes with alkyl or aryl substituents in the ring probably proceeds by a rhombic twist mechanism. Those complexes with a fluorocarbon ring substituent probably stereoisomerize by a bond-rupture mechanism with a square-pyramidal intermediate with the unidentate /9-diketonate ligand in the apical position. 

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