The achiral coordination polymer [Mn(II)(rac-pnH)(H(2)O)Cr(III)(CN)(6)].H(2)O, (rac-pn = racemic 1,2-diaminopropane), 1.2H(2)O, has been prepared, and its crystal structures, optical and magnetic properties have been studied before and after dehydration followed by rehydration. The in situ X-ray diffraction, performed on one selected single crystal, shows an unusual irreversibility from the as-prepared 1.2H(2)O to the dehydrated [Mn(rac-pnH)Cr(CN)(6)], 1, and reversibility from 1 to rehydrated [Mn(rac-pnH)(H(2)O)Cr(CN)(6)].H(2)O 1-HP. Virgin 1.2H(2)O crystallizes in the monoclinic achiral P2(1)/m space-group having a two-dimensional (2D) square-network of Mn-Cr with bridging cyanide, and behaves as a soft ferrimagnet (T(C) = 36 K). Dehydrated 1 has a three-dimensional (3D) network with an additional cyanide bridge between layers and adopts the orthorhombic achiral Pmnb space-group exhibiting a ferrimagnetic behavior (T(C) = 70 K). Rehydrated 1-HP (T(C) = 36 K) is poorly crystallized having the same unit-cell as 1.2H(2)O and reversibly transforms to the crystalline 1 (T(C) = 70 K). The dehydration is associated to a change in the coordination of the amine from one layer to its neighboring one involving a proton transfer, going from {Mn-NH(2)CH(CH(3))CH(2)N'H(3)(+).....(H(2)O)Mn'} for 1.2H(2)O to {Mn-NC.....NH(3)(+)CH(CH(3))CH(2)H(2)N'-Mn'} for 1. The irreversible transformation of virgin single crystal 1.2H(2)O to single-crystal 1 is promoted by the availability of only one Mn in the vicinity of the cyanide while during the rehydration process the reversible single-crystal 1 to a glassy 1-HP is due to the presence of two equidistant Mn atoms, which is the cause of the disorder. The change in magnetism, that is, the increase of the Curie temperature and coercive field, is associated to the structural transformation from 2D to 3D.