The title compound, K2Mn3(H2O)6 [Mo(CN)(7)](2) . 6H(2)O, has been synthesized by slow diffusion of aqueous solutions saturated in KNO3 and containing K-4[Mo(CN)(7)]. 2H(2)O and [Mn(H2O)(6)](NO3)(2), respectively. The compound crystallizes in the monoclinic system, space group C2. The Mo site is surrounded by six -C-N-Mn linkages and one terminal cyano group in a strongly distorted pentagonal bipyramid fashion. There are two distorted octahedral Mn sites, both with four -N-C-Mo linkages and two water molecules in trans conformation. The structure is two-dimensional with anionic layers perpendicular to the a axis and K+ ions located between the layers. Each layer is constituted of two parallel gridlike sheets made of edge-sharing Mo2Mn2 lozenge motifs, connected by Mn(CN)(4)(H2O)(2) units situated between the sheets. Thorough magnetic investigations have been carried out. First, it has been demonstrated that the magnetic axes are collinear with the a, b, and c* crystallographic directions. The temperature and field dependences of the magnetization in both the de and ac modes have been measured along the principal axes. The angular dependence of the magnetization in the bc* plane as a function of the external field has also been measured. These investigations have revealed that the compound orders ferromagnetically at T-c= 39 K, with a small hysteresis effect along the c* axis. Furthermore, a field-induced spin reoffentation has been detected. The b axis is the easy magnetization axis in low field, and the bc* plane is very anisotropic. When the field reaches a critical value, the bc* plane becomes almost isotropic. From the magnetic data, the magnetic phase diagram has been determined. When a single crystal of K2Mn3(H2O)(6)[Mo(CN)(7)](2). 6H(2)O is partially dehydrated under vacuum, its shape remains unchanged, but T-c is shifted up to 72 K, and the spin reorientation is suppressed. The partially dehydrated material presents a coercive field of 1.3 kOe along b at 5 K. These findings have been compared to those reported recently for a three-dimensional material of the same family (the alpha-phase Mn-2(H2O)(5)-Mo(CN)(7) . 4H(2)O).