[jgtttt]

Why does the addition of DMSO to the red form of cobalt salen activate it whereas chloroform to the oxygenated complex deactivates it?

[Flatbutterfly]

“The student can be asked to discuss the factors which affect the equilibrium in DMSO and chloroform solvents
2Co(salen) + 2DMSO + O2 ⇋   [(DMSO)Co(salen)]2O2
Electron donation by a ligand, L, is required to stabilize the structure.  Chloroform is a poor donor unlikely to coordinate to cobalt. In DMSO, the high concentration of DMSO provided by the solvent, and the low solubility of the adduct help to force the equilibrium to the right.”
Oxygen uptake by a Co(II) Complex  T. G. Appleton J. Chem. Ed. 1977, 54, 443

My view: the DMSO alters the HOMO-LUMO properties of the Co(II) such that there is better overlap with the frontier MOs on O2 (a trans influence).  O2 bonds to Co at an angle via the π* MOs [Co(III) – O2(-)]