The key C–C bond forming step of the Rh-catalysed carboxylation of organoboron compounds with CO₂ was investigated by density functional theory (DFT) at the IEFPCM/PBE0/DGDZVP level of theory. With a bidentate phosphane (dppp) ligand, the reaction begins with a transformation from square-planar O-(η¹)-bonded to distorted-tetrahedral, η²-bonded CO₂ followed by migratory insertion (barrier 12.7 kcal mol⁻¹). The calculations showed that incorporation of electron-donating substituents within either the aryl substrate or the phosphane leads to a slight decrease of the reaction barrier, in excellent agreement with the experimental trends. With a diene ligand (cod), the migratory insertion proceeds directly from the O-(η¹)-bonded CO₂ (barrier 17.1 kcal mol⁻¹) as a consequence of the diminished σ-donor ability of the diene ligand. Therefore, the diene ligand is predicted to be an inferior choice compared to the diphosphane. Testing of [(cod)Rh(OH)]₂ upon completion of the computations showed no conversion.