The trans-Mn III (Salophen)(OH 2) 2 + and bioxalate (HOX −) in aqueous medium equilibrate rapidly to trans-Mn III (Salophen)(OH 2)(HOX) followed by the acid dissociation equilibrium to the (aqua) mono oxalato complex. The slow redox reactions of trans-Mn III (Salophen)(OH 2)(HOX/OX) 0/− with H 2 OX, HOX − ,OX 2− obey second order kinetics satisfying 2:1 stoichiometry ([Mn III ] T /[OX] T = 2/1). The products are Mn II and CO 2. Acrylamide monomer has no effect on the rate constant and the reaction does not induce its polymerization. The rate and activation parameters for the various rate limiting paths are reported. The intramolecular reduction of Mn III by the coordinated HOX − and OX 2− in trans-Mn III (Salophen)(OH 2)(HOX/OX) 0/− could not be detected. Contrary to our expectation, it is observed that H 2 OX is a better reducing agent than HOX − for trans-Mn III (Salophen)(OH 2)(HOX), the slowest being the redox reaction of OX 2− with trans-Mn III (Salophen)(OH 2)(OX) −. The molecular modelling by DFT depicts the structural trans effect in the oxalato complexes, it being maximum for trans-Mn III (Salophen)(OH 2)(OX) −. The observed sequence of the redox activity of the oxalato complexes reflects the potential role of non-covalent interaction i.e. H-bonding, governing the proton controlled electron transfer process (PCET). The Mn III (Salophen/Salen) complexes may turn out to be good substitute candidates for Oxalo Oxidase (OXO) enzyme in alleviating the oxalate overload in plants and animal biochemistry.

Water exchange of trans-[MnIII(salen)(OH2)2]+ studied by line broadening 17OH2 NMR discloses its mechanism as associative interchange (Ia).