In summary, we have shown two Structurally similar Mn-oxo clusters, Mn12 and Mn13, exhibit distinct magnetic properties: Mn13 is an SMM, yet Mn12 is not. Both were synthesized under the same conditions except for employing different solvent (Mn12: THF/CH₂Cl₂, Mn13: MeCN/CH₂Cl₂). ESI-MS explains the details of the reaction process and reveals the corresponding solvents acts as a shield to protect the corresponding clusters, leading to eventual crystallization, while both the clusters along with minor byproduct forms a chemical equilibrium among them in the real time solution phase. Theoretical calculations of the energy states also corroborated the experimental finding as increasing THF solvent molecule leads to the formation of hydrogen bonding which makes Mn12 reluctant to undergo dehydration to form Mn13 in solution phase and hence the chemical equilibrium shifts towards the more stable Mn12 component and eventually crystallizes and similarly Mn13 forms with a gradual increase of MeCN solvent. This work not only explains the basic principle of cluster crystallization process, but also provides us in-depth analysis of course of crystallization in solution phase and sheds light on a new way to synthesize molecule-based magnets with subtle optimization of solvent molecules.