Potassium by Thallium Substitution: Symmetry Transformation Through Site Competition in Coordination Compounds

The study focused on the nitrilo‐tris‐(methylene phosphonate) [K2ntpH4]2 and [(K1−yTly)2ntpH4]2 complexes, which are a series of isovalent substitutions. The crystals of [K2ntpH4]2 are monoclinic and belong to the space group of symmetry P21/c with Z=4. When thallium substitutes potassium, the molecular structure remains the same, but the symmetry of the crystal packing changes to a triclinic system, space group of symmetry P 1‾ ${\bar{1}}$ , with Z=2. One coordination polyhedra maintains an isometric configuration, while the other undergoes significant distortions. Polarization‐selective coordination is observed, where the thallium(I) ion, which is easily polarizable, populates the highly distorted polyhedron. The low polarizable potassium ion populates the weakly distorted one. The crystal is formed with an average thallium fraction higher than the initial aqueous solution. This suggests that replacing potassium ions in the complex with thallium ions is energetically favourable. It could explain why thallium is toxic to intracellular enzymes and carriers of genetic information. The isovalent substitution of potassium made it possible to observe the step‐by‐step formation of a super‐strong hydrogen bond, a phenomenon that is rarely detected under normal conditions. The O…O interatomic distance in the hydrogen bond decreases gradually from 2.55 Å for [K2ntpH4]2 to 2.474 Å for [Tl2ntpH4]2. The study investigates the composition and structure of coordination complexes of monovalent metals with nitrilo‐tris(methylene phosphonic acid). The findings show that when potassium is replaced with thallium, the Tl : K ratio in the PO3 coordination environment is consistently higher than in the solution. It was found that thallium occupies only half of the coordination sites, decreasing the local symmetry of its coordination environment due to its high polarizability compared to low polarizable potassium.