The equilibrium between the radical phenanthroline complex Cp*2 Sm(phen) and the coupling product (Cp*2 Sm(phen))2 has been investigated based on quantum chemistry calculations. Topological analyses pointed out that the C-C bond created has a partial covalent character, explaining why both the monomeric and the dimeric forms exist in equilibrium. A large variety of density functionals have been tested to reproduce experimental thermodynamic data for this equilibrium. Finally, the PBE0-D3 and M06-2X functionals lead to a good evaluation of the energies and enable a correct description of the ligand to metal charge transfer, both in the 4f and 5d metal orbitals.