Bailar inversion

Even though the activation energy of this process may be less than that for other paths, a low frequency factor would allow other paths to compete at higher temperatures. This postulate would also explain the temperature dependence of the Bailar inversions. An alternate, oriented ion-pair mechanism has been suggested previously (P, 26 6S) but the observations with [CoCl2(en)2] in methanol ( 21 make this less probable because both the ion pair and the solvated ion appear to give trans and racemic cis isomers under the conditions studied. Some orientation effects would be expected for the ion-pair reaction if it were operative in the inversion reactions. 

Although the Bailar inversion (an octahedral complex substitution D L conversion, where d represents a right-handed screw relative to the C2 symmetry axis of an octahedral complex ion of the type cis-[Co(en)2Cl2] in which en = ethylenediamine, as shown in Fig. 1) was first reported by John C Bailar, Jr. and R. W. Auten in 1934, the exact mechanism of this type of reaction is still unknown. Originally, the reaction was considered to be due to the reaction of Ag2C03 with the [Co(en)2Cl2] ion... 

Most of the inversion reactions related to the above Bailar inversion have also been studied either by Bailar, his students, and his students students, or by the capable Australian chemists following in the footsteps of the late Frank Dwyer. Bailar has reviewed the contributions made at the University of Illinois at a Dwyer memorial lecture. ... 

The reaction between D -[Co(en)2Cl2] and ammonia also shows a Bailar inversion. Correlation of the appropriate optical rotatory dispersion curves and the temperature dependence of the optical rotatory dispersion... 

No other Bailar inversions have been observed for any other complexes in which the coordination sphere is occupied by only monodentate and bidentate ligands, or for complexes with other metal ions. On the other hand, a sizable number of new d - l inversions have been observed which appear to be related to the steric restraints of polydentate ligands. 

In order to more fully elucidate the Bailar inversion mechanism before considering other systems which exhibit sterochemical changes during substitution, Elaine Dittmar, one of my students, used a combination of visible spectrophotometry, polarimetry, and ion-exchange chromatography... 

The involvement of conjugate base species in such base hydrolysis reactions must be considered in arriving at a mechanistic path for the Bailar inversion. And even though a d and l inversion can be accommodated by an unsymmetrical trigonal bipyramidal conjugate base intermediate, " the concentration and silver ion dependence of the Bailar inversion require other explanations. Four such possibilities can be imagined for the concentration dependent reaction. 

These possible mechanistic paths have been evaluated elsewhere in some detail. Elucidation of the stereochemistry of a Bailar inversion with an unsymmetrical diamine would distinguish between the trans attack mechanism and the other possibilities. 

Circular dichroism has been used to study the base hydrolysis of the ri.-[Co(en)2-CI2I+ ion, a reaction which proceeds in a concerted way with Bailar inversion ... 

Concerted base hydrolysis and Bailar inversion of the /l-cis -[Co(en)2Cl2]+ ion has been established by a circular dichroism study [equation (18)]. Base hydrolysis of the /l-c/j-[Co(en)aCl(OH)]+ ion is reported to proceed with complete retention of configuration in contrast to an earlier investigation of this reaction. The results of these studies were considered previously. ... 

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