The transition structures (TSs) for the carbon-carbon bond-breaking step in the carboxylation and oxygenation reactions catalyzed by Rubisco are studied at HF/6-31G** and full MP2/6-31G** levels. The active atoms of the substrate (ribulose 1,5-bisphosphate) are modeled with 3- and 5-carbon atom molecules. A model coordination sphere is included in order to detect the role of magnesium on the TS. The electron localization function ((ELF) is used to obtain the basin structure of the transition structures, giving a pictorial description of the wave function allowing for a distinction between heterolytic-like and homolytic-like mechanisms for the C2-C3 breaking process.