Kinetics and mechanism of formation, acid catalysed aquation, reversible anation and photochemical reaction of trans-(aqua)(sulfito-S)[N,N '-ethylenebis(salicylidiniminato)]cobaltate(III) in aqueous media

The reaction of trans-[Co(salen)(OH2)OH] with SO2 yields trans-[Co(salen)(O H2)(SO3-S)](-) (S-bonded isomer) for which the rate and activation paramete rs at 25 degrees C (I=0.3 mol dm(-3)) are k(SO2)=(5.9 +/- 0.1)x10(10) dm(3) mol(-1) s(-1), Delta H double dagger=66 +/- 4 kJ mol(-1) and Delta S doubl e dagger=183 +/- 14 J K-1 mol(-1). One possibility for the S-IV substitutio n is that Co-S bond formation is concerted with Co-O bond breaking. An alte rnative mechanism, involving a fast equilibrium between SO2 and trans-[Co(s alen)(OH2)OH] forming an O-bonded sulfito species which then undergoes sulf ite ligand linkage isomerisation, is also possible. An estimated value of t he isomerisation rate constant for the trans-[Co(salen)(OH2)(OSO2H)] at 25 degrees C is ca. 10(6) s(-1). The trans-[Co(salen)(OH2)(SO3-S)](-) (pK=10.1 +/- 0.1 at 25 degrees C, I=0.3 mol dm(-3)) undergoes acid catalysed aquati on to yield the parent diaqua complex and S-IV with k(H)=29.5 +/- 1.1 dm(3) mol(-1) s(-1), Delta H double dagger=72 +/- 3 kJ mol(-1), Delta S double d agger=24 +/- 9 J K-1 mol(-1) at 25 degrees C (I=0.3 mol dm(-3)). Steady sta te photolysis (254 nm) of trans-[Co(salen)(OH2)(SO3-S)](-) resulted in the reduction of Co-III. The redox rate constant and phi(Co2+) decreased with i ncreasing pH. Attempts to detect an O-bonded sulfito complex as a transient in the conventional flash photolysis of this aqua-sulfito complex proved u nsuccessful. The aqua ligand replacement reactions of trans-[Co(salen)(OH2/OH)(OH2)](+/0 ) with imidazole and that of the corresponding aqua-sulfito complex with N- 3(-), NCS-, imidazole, and S-IV in a large excess of the entering ligands h ave been studied at 25 degrees C. A comparison of the rate constants with t he analogous data for trans-[Co(AA)(2)(OH2)(SO3-S)](+) (AA=1,2-diaminoethan e; 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) ](-).