Interfacial nonradiative recombination limits tin-lead perovskite photovoltaics. Here we show that neutral ethylenediamine chelates undercoordinated metals at tin-rich surfaces, weakens metal-iodide bonds, and removes excess tin, reconstructing the surface towards a stoichiometric tin-to-lead ratio. In contrast, protonated ethylenediammonium iodide initially passivates undercoordinated metal sites before undergoing defect-assisted deprotonation. Atomistic simulations reveal that subsurface iodide vacancies promote iodide migration and lower the energetic cost of deprotonation. Ethylenediamine-induced reconstruction produces stronger n-type surface character, facilitates electron extraction into C60, and suppresses interfacial nonradiative recombination, enabling methylammonium-free solar cells with a power conversion efficiency of 25.86%. Together, these distinct surface reaction pathways and the defect-assisted evolution of the passivating species underscore the importance of understanding perovskite surface chemistry for the rational design of molecular passivation strategies.