Theoretical study of the formation of Pu^Z+ (Z = 3, 4, 5, 6) complexes with the two types of diamide molecules C₈N₂H₁₀O₂(CH₃)₂ (dimethylbicyclicdiamide-DMDA) with bicyclic structure and C₃N₂H₂O₂(CH₃)₄ (tetramethylmalonamide-TMMA) with acyclic structure was carried out using ab initio DFT based DMol³ and relativistic discrete variational methods. The results of gas phase modeling showed that the appropriate positions of the oxygen atoms in DMDA in comparison with TMMA cannot explain the considerable difference in sorption affinity of these molecules. For the modeling of solutions, the implicit (COSMO) and explicit methods were used. In the latter approach, we included 40 water molecules, 16NO₃^- or 16Cl^- and 16H⁺ ions into the systems under investigation. The obtained results showed the principle role of solution in the weakening of actinide bonding with TMMA and DMDA molecules due to interaction with H₂O and NO₃^- or Cl^- ions.