Pretreatment using metal chlorides during the formation of halide deficient free perovskite nanocrystals is reported. Among several metal chlorides, Cu(II)Cl2 was observed to be ideal for the synthesis of highly emitting CsPbCl3 nanocrystals at high reaction temperature. Because high temperature remained more favorable for the dopant insertion, doping of Mn(II) was carried out under this halide-rich system, and nearly 68% photoluminescence quantum yield was recorded. Analysis could not provide strong evidence of insertion of Cu(II) inside the nanocrystals; rather, it was established that Cu(II)Cl2 in the system helped to stabilize the reaction even at and above 260 °C and provided an adequate chloride source for obtaining the highly emitting host as well as doped nanocrystals. Details of the physical process involved for this metal ion-induced uplifting of the reaction temperature and the consequent impacts on the nanocrystal formation are studied in detail and reported in this Letter.