In this work we present a comprehensive study of the influence of the solvent on the post-synthetic modification (PSM) on the [Fe(NH₂trz)₃](NO₃)₂ spin crossover (SCO) complex with p-anisaldehyde. We demonstrate that the use of a suitable solvent such as ethanol results in a complete PSM, where the final crystalline material exhibits a gradual conversion just below room temperature. In the other hand, when an unsuitable solvent such as toluene or 1-octanol is used, either no reaction or very long reaction time is needed, leading to undesired Fe(III) species. Transmission electronic microscopy analyses show that the complete PSM reaction carried out on nanoparticles of the same complex, changes drastically the size and morphology of the particles.