We study, for the first time the, structure of Stable finite size clusters (i.e., colloidal molecules) obtained by self-assembly of cationic gold nanoparticles (i.e., atoms) mediated by a flexible polyanion We reveal with non-denaturizing techniques a striking structural: transition from ID small chains of 12 gold nanoparticles (AuNPs) with a self-avoiding conformation to 3D fractal clusters of 130 AuNPs with short-range. ordering around the charge inversion threshold. Interestingly, these well-defined structures are obtained by simple mixing in water without anisotropic functionalization or external forces, As a preliminary step, We introduce a new synthesis pathway leading to well-defined cationic AuNPs of controllable size that can be dispersed in H2O or D2O without aggregation and ligands' self-assemblies. On tin's occasion, we point for the first time that usual procedures do not enable to eliminate cationic ligands' self-assemblies that could play an undesired role in AuNPs' self-assembly through electrostatic interactions.