The method of molecular mechanics has been used to calculate the strain energy of complex formation of two new phosphorus-organic cyclopendant ligands, derivatives of 1,4,7-triazacyclononane. It is shown that lengthening the pendant groups by one CH₂ unit increases the volume of the ligand cavity in hexadentate coordination. However, this requires a large strain energy, which leads to a loss of stability of the complexes with metal cations. A decrease in the strain energy for penta- and tetradentate ligand profiles accounts for the appearance of selectivity towards cations with small radii. Modifying the structure of cyclopendants based on triazacyclononane by means of benzene rings does not produce a ligand profile with a definite structure, or selective binding of metal cations.