[NH4](2)Mn-3(H2O)(4)[Mo(CN)(7)](2).4H(2)O (1) has been synthesized by slow diffusion of aqueous solutions containing K-4[Mo(CN)(7)].2H(2)O, [Mn(H2O)(6)](NO3)(2), and (NH4)NO3. Compound 1 crystallizes in the monoclinic C2/c space group. The basic motif of the three-dimensional structure consists of a Mo1-Mn1 gridlike sheet parallel to the bc plane. Two of these sheets are connected through CN-Mn2-NC linkages to form a bilayer reminiscent of the K2Mn3(H2O)(6)[Mo(CN)(7)](2).6H2O (2) two-dimensional structure. In 1, [NH4](+) cations allow these bilayers to be connected through direct Mo1-CN-Mn1 bridges to form a three-dimensional network, whereas in 2, they are isolated by (H2O)K+ cations. As shown by the magnetic measurements, this increase of dimensionality by counterion substitution induces an enhancement of the ferrimagnetic critical temperature from 39 K in 2 to 53 K in 1.