Eigenanalysis of the Coupled First-Order Isomerization Reactions of the Bis[(S)-alaninato]dinitrocobaltate(III) Ion

Abstract

Three of the five possible geometric isomers of the bis[(S)-alaninato] dinitrocobaltate (III) ion isomerize to the stable fourth isomer by a series of first-order reactions. The fifth isomer is not observed. The kinetics of the isomerizations in neutral, aqueous solutions between 65 and 95 C have been studied by chromatographic separation of the isomers. The data were fitted by eigenanalysis of the coupled first-order differential equations. The results are consistent with a reaction scheme requiring five rate constants, k_xy, where xy represents isomerization from isomer x to isomer y. lsomer A is the Λ-(-)₅₈₉-cis-NO₂/cis-N/cis-O complex; isomer B the trans-NO₂/cis-N/cis-O complex; isomer C the trans-NO₂/trans-N/trans-O complex; and isomer D is the mixture of both diastereomers of the cis-NO₂/trans-N/cis-O complex that is formed from B and C. The rate constants required are k_AB, k_bc , k_BD , k_CB and k_CD . The same reaction scheme had been found previously for the analogous glycinato complexes, but the activation energies differ. For the valine and leucine analogues, the rate constants and activation enthalpies for the first isomerization reaction, represented by k_AB, are similar to that of the alanine complex.