Kinetics and mechanism of reaction of trans-(diaqua)(N,N'-ethylene-bis-salicylidenimine) chromium(iii) with sulfur(iv): the labilizing effect of coordinated sulfite

Abstract

The reaction of trans-[Cr(Salen)(OH₂)₂]⁺with aqueous sulfite yields trans-[Cr(Salen)(OH₂)(OSO₂(SINGLEBOND)O)]^- (O-bonded isomer). The rate and activation parameter data for the formation of the sulfito complex are consistent with a mechanism involving rate-limiting addition of SO2 to the Cr^III(SINGLEBOND)OH bond. The complex ions, trans-[(OH₂)Cr(Salen)(OSO₂(SINGLEBOND)O)]^-, and trans-[(OH)Cr(Salen)(OSO₂(SINGLEBOND)O)]^2-, undergo reversible anation by NCS^-, N^3-, imidazole, and pyridine resulting in the formation of trans-[XCr(Salen)(OSO₂(SINGLEBOND)O)]^(N+1)-(n=1 for X=N₃^-,NCS^-, and 0 for X=imidazole and pyridine) predominantly via dissociative interchange mechanism. The labilizing action of the coordinated sulfite on the trans-Cr^III-X bond in trans-[XCr(Salen)(OSO2)]^(n+1)- follows the sequence: NCS^-pyridine ca. N₃^- ca. imidazole. Data analysis indicated that the coordinated sulfite has little trans activating influence.