The reaction of trans-[Co(salen)(OH2)OH] with SO2 yields trans-[Co(salen)(OH2)(SO3-S)]− (S-bonded isomer) for which the rate and activation parameters at 25 °C (I = 0.3 mol dm−3) are kSO2 = (5.9 ± 0.1) × 1010 dm3 mol−1 s−1, ΔH‡ = 66 ± 4 kJ mol−1 and ΔS‡ = 183 ± 14 J K−1 mol−1. One possibility for the SIV substitution is that Co–S bond formation is concerted with Co–O bond breaking. An alternative mechanism, involving a fast equilibrium between SO2 and trans-[Co(salen)(OH2)OH] forming an O-bonded sulfito species which then undergoes sulfite ligand linkage isomerisation, is also possible. An estimated value of the isomerisation rate constant for the trans-[Co(salen)(OH2)(OSO2H)] at 25 °C is ca. 106 s−1. The trans-[Co(salen)(OH2)(SO3-S)]− (pK = 10.1 ± 0.1 at 25 °C, I = 0.3 mol dm−3) undergoes acid catalysed aquation to yield the parent diaqua complex and SIV with kH = 29.5 ± 1.1 dm3 mol−1 s−1, ΔH‡ = 72 ± 3 kJ mol−1, ΔS‡ = 24 ± 9 J K−1 mol−1 at 25 °C (I = 0.3 mol dm−3). Steady state photolysis (254 nm) of trans-[Co(salen)(OH2)(SO3-S)]− resulted in the reduction of CoIII. The redox rate constant and ϕ(Co2+) decreased with increasing pH. Attempts to detect an O-bonded sulfito complex as a transient in the conventional flash photolysis of this aqua-sulfito complex proved unsuccessful.
The aqua ligand replacement reactions of trans-[Co(salen)(OH2/OH)(OH2)]+/0 with imidazole and that of the corresponding aqua-sulfito complex with N3−, NCS−, imidazole, and SIV in a large excess of the entering ligands have been studied at 25 °C. A comparison of the rate constants with the analogous data for trans-[Co(AA)2(OH2)(SO3-S)]+ (AA = 1,2-diaminoethane; 1,3-diaminopropane) clearly shows that the kinetic trans-effect of the S-bonded sulfite is substantially attenuated in trans-[Co(salen)(OH2)(SO3-S)]−.