Dynamic molecular crystals are of high interest due to their potential applications. Herein we report the reversible on–off switching of single-molecule magnet (SMM) behavior in a Mo­(CN)74– based molecular compound. Upon dehydration and rehydration, the trinuclear Mn2Mo molecule Mn­(L)­(H2O)2Mo­(CN)7·2H2O (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear Mn4Mo2 compound Mn­(L)­(H2O)2Mn­(L)2Mo­(CN)72 (2). This structural transformation involves the breaking and reforming of coordination bonds which leads to significant changes in the color and magnetic properties. Compound 1 is an SMM with an energy barrier of 44.9 cm–1, whereas 2 behaves as a simple paramagnet despite its higher ground state spin value. The distortion of the pentagonal bipyramidal geometry of Mo­(CN)74– in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.