The Chemistry From Tin Iodide Molecular Inks to FASnI₃ Nanocrystals

The controlled synthesis of high-performance tin halide perovskite nanostructures hinges on the coordination chemistry that governs precursor speciation within molecular inks. Here, we elucidate the complexation dynamics of SnI₂ with two benchmark Lewis bases widely used in nanocrystal syntheses: a strong primary amine (R-NH₂) and a weaker substituted phosphine (R'₃-P). Correlated in situ ^{1 1 9}Sn NMR and UV-Vis absorption spectroscopy, supported by density functional theory calculations, reveal that both ligands (L) form monomeric SnI₂-L adducts, with R-NH₂ consistently exhibiting stronger coordination than R₃'-P, as quantified by the intrinsic bond strength index and interaction energies. Higher ligand loadings destabilize SnI₂-L_x complexes, particularly for phosphines, whereas we show that amine-bound multimeric (SnI₂)_x(R-NH₂)_x (x = 2-3) species can be present at low ligand concentrations. These molecular-level insights directly correlate with nanocrystal formation pathways. Stronger Sn─N coordination drives the emergence of 2D Ruddlesden-Popper phases, while weaker Sn-P interactions favor bulk-like 3D FASnI₃ nanocrystals due to insufficient stabilization of early-stage intermediates. Guided by this understanding, an amine-free, three-precursor strategy employing a strong zwitterionic ligand enables phase-pure 3D FASnI₃ nanocrystals with improved optical and colloidal stability. This work establishes a predictive framework for designing robust molecular inks for tin halide perovskites and perovskitoid nanostructures.