Nonclassical crystallization of metallic nanoparticles through cluster collapse and reassembly.
Crystallization from solution is commonly described in terms of direct nucleation from dispersed monomeric species, yet this picture becomes inadequate when strong coordination or solvation suppresses free-particle formation. Here we resolve the growth pathway of Pt nanoparticles formed by CO-mediated reduction of Pt precursors using real-time mass spectrometry, infrared and UV-vis spectroscopy, ultrafast chemical exchange measurements, and theory. We show that classical monomer-based nucleation is bypassed: strong Pt-CO coordination stabilizes metastable multinuclear clusters, with [Pt3(CO)6]n2- (n = 4) emerging as a dominant intermediate. Rather than acting as seeds, these clusters undergo a collapse-reassembly process upon oxidation, in which partial ligand loss generates locally concentrated, coordinatively unsaturated Pt fragments that rapidly reorganize into crystalline nanoparticles, enabling growth without the entropic penalty of stochastic atom aggregation. This mechanism may be broadly relevant to nanoparticle formation under strong coordination and redox-active conditions, and may also extend to crystallization processes in strongly solvated systems like ions in aqueous solutions, where transient, non-equilibrium clusters mediate the transition from molecular precursors to extended crystalline matter.