Mechanistic Studies on the reactions of cis‐diaquobisoxalatochromate(III) ion with 2,2′‐dipyridyl and 1,10‐phenanthroline

Kinetic studies in aqueous solutions on the replacement of the aquo ligands in cis‐Cr(Ox) 2 (H 2 O)   2 −by 2,2′‐dipyridyl (dipy) and 1,10‐phenanthroline (phen) forming cis‐Cr(Ox) 2 (AA) − (AA = dipy or phen) were made spectrophotometrically. The reaction in each case occured in two concurrent paths, one of which was independent of the reagent (AA) concentration (rate constant, k 0 , identical for both the systems), and another which was first order with respect to it (rate constant, k R ). k 0 and k R values have been evaluated at different temperatures (40–70°C), and from there the corresponding ΔH≠ and ΔS≠ values. The results suggest a dissociation mechanism for the reagent independent path where CrOH 2 bond rupture is only significant in the transition state. The value of k R /k 0 (higher for phen compared to dipy) was of the order of 10 2 suggesting significant bond formation by the reagent in the transition state of the reagent dependent path. However, ΔH≠ corresponding to k R was ca. 1.7 to 1.8 times that corresponding to k 0 indicating that in the reagent dependent path simultaneous rupture of the two CrOH 2 bonds in cis positions occur as expected from steric considerations for these bidendate ligands. Increase in ionic strength of the of the medium causes a slight acceleration of the reagent‐dependent path only.