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

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⁻, N₃⁻, 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. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 373–384, 1998