The Chemistry From Tin Iodide Molecular Inks to FASnI 3 Nanocrystals

ABSTRACT 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 SnI 2 with two benchmark Lewis bases widely used in nanocrystal syntheses: a strong primary amine (R–NH 2 ) 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 SnI 2 –L adducts, with R–NH 2 consistently exhibiting stronger coordination than R 3 ′–P, as quantified by the intrinsic bond strength index and interaction energies. Higher ligand loadings destabilize SnI 2 –L x complexes, particularly for phosphines, whereas we show that amine‐bound multimeric (SnI 2 ) x (R–NH 2 ) 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 FASnI 3 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 FASnI 3 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.