¹ H NMR chemical shift concentration dependence as well as the diffusion coefficients from DOSY NMR of mixed ligand [Pt^II(1,10-phenanthroline)(N-pyrrolidyl-N-(2,2-dimethylpropanoyl)thiourea)]Cl ([Pt^II(phen)(L¹-S,O)]Cl) dissolved in mixtures of acetonitrile–water in the range 0–30% (v/v) D₂O–CD₃CN shows that the complex cation (M⁺ = [Pt^II(phen)(L¹-S,O)]⁺) aggregates to form dimers, 2M⁺ {M⁺}₂, with association constants ranging from K_D(CD₃CN) = 17 ± 2 M⁻¹ to K_D(30% (v/v) D₂O–CD₃CN) = 71 ± 8 M⁻¹ at 299.3 K, presumably via non-covalent cation–π interactions. Experimental data are consistent with an ‘offset’ face-to-face cation–π stacking arrangement of the planar cation. However in water-rich solvent mixtures from >30% (v/v) D₂O–CD₃CN to pure D₂O, the extent of aggregation significantly increases until a critical aggregation concentration (CAC) is reached, estimated to be 9.6 and 10.3 mM from ¹H NMR chemical shift concentration dependence and DOSY NMR measurements respectively. Above the CAC the formation of nano-structures formulated as {[Pt^II(phen)(L¹-S,O)]⁺}_nCl⁻_y (n, y > 2) is indicated. DOSY studies show a significant decrease of the average diffusion coefficient D_obs as a function of increasing concentration of [Pt^II(phen)(L¹-S,O)]Cl in D₂O. The aggregation number (N) estimated from hydrodynamic volumes of the mononuclear [Pt^II(phen)(L¹-S,O)]⁺ cation (V⁰_H), and those V_H estimated from D_obs (N = V_H/V⁰_H) as a function of total complex concentration, ranges from 2 to 735 in pure D₂O. Above the CAC well defined nano-structures which may be loosely termed “metallogels” could be characterized by means of transmission electron microscopy. As expected the addition of NaCl appears to increase the extent of aggregate formation, by presumably stabilizing the formation of nano-sized {[Pt^II(phen)(L¹-S,O)]⁺}_nCl⁻_y aggregates preventing excessive positive electrostatic charge build-up.