A mononuclear cobalt(III) carboxylate complex with a fully O-based coordination sphere: Co <superscript>III</superscript> -O bond homolysis and controlled radical polymerisation from [Co(acac) <subscript>2</subscript> (O <subscript>2</subscript> CPh)].

The addition of benzoyl peroxide to [Co II (acac) 2 ] in a 1 : 2 ratio selectively produces [Co III (acac) 2 (O 2 CPh)], a diamagnetic (NMR) mononuclear Co III complex with an octahedral (X-ray diffraction) coordination geometry. It is the first reported mononuclear Co III derivative with a chelated monocarboxylate ligand and an entirely O-based coordination sphere. The compound degrades in solution quite slowly by homolytic Co III -O 2 CPh bond cleavage upon warming above 40 °C to produce benzoate radicals and can serve as a unimolecular thermal initiator for the well-controlled radical polymerisation of vinyl acetate. Addition of ligands (L = py, NEt 3 ) induces benzoate chelate ring opening and formation of both cis and trans isomers of [Co III (acac) 2 (O 2 CPh)(L)] for L = py under kinetic control, then converting quantitatively to the cis isomer, whereas the reaction is less selective and equilibrated for L = NEt 3 . The py addition strengthens the Co III -O 2 CPh bond and lowers the initiator efficiency in radical polymerisation, whereas the NEt 3 addition results in benzoate radical quenching by a redox process. In addition to clarifying the mechanism of the radical polymerisation redox initiation by peroxides and rationalizing the quite low efficiency factor for the previously reported [Co II (acac) 2 ]/peroxide-initiated organometallic-mediated radical polymerisation (OMRP) of vinyl acetate, this investigation provides relevant information on the Co III -O homolytic bond cleavage process.