MECHANISM OF METAL ION PROMOTED DISSOCIATION OF SOME OXALATO COMPLEXES OF CHROMIUM(III)

Abstract

Abstract Dissociation of oxalato complexes of chromium (III) of the type [Cr(ox)n (OH2)6–2n](3–2n)+ into [Cr(ox)n-1 (OH2)8–2n](5–2n)+ (where n = 1, 3) promoted by different metal ions of the first transition series in acid perchlorate media has been investigated spectrophotometrically. In comparable cases the order of the catalytic activity as measured by the second-order rate constant kM, follows the sequence Fe3+ >> Cu2+ > Ni2+ > Zn2+ > Co2+ > Mn2+. Log kM for a particular substrate complex and different promoter metal ions of same charge varies linearly with log KM-ox, KM-ox being the formation constant of the mono-oxalato complex of the catalysing metal ion Mn+. The value of kH+ for the acid catalysed reaction occupies a position intermediate between kCo2+ and kMn2+ and even for the positively charged complex [Cr(ox) (OH2)4]+, Fe3+ is a far better catalyst than H+, indicating the importance of specific bonding between the promoter and the substrate complex in the intermediate step. These conclusions are corroborated by various free energy relationships and also by comparing results of the present work with literature data on other similar systems. Reactions of a particular substrate complex as promoted by different metal ions and also that by H+ are all practically isoenthalpic.