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)(7)](4-) based molecular compound. Upon dehydration and rehydration, the trinuclear Mn2Mo molecule [Mn(L)(H2O)](2)[Mo-(CN)(7)]center dot 2H(2)O (1) undergoes reversible crystal-to-crystal transformation to a hexanuclear Mn4Mo2 compound [Mn(L)(H2O)](2)[Mn(L)](2)[Mo(CN)(7)](2) (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)(7)](4-) in 2 disrupts the anisotropic exchange interactions that lead to SMM behavior in 1.