The flexibility offered by molecular-based systems allows us to introduce or replace specific ligands in a material with the aim of radically altering desired structural and magnetic properties. Specifically, Mn(dca)₂(pyz) {dca = dicyanamide, [N(CN)₂]^-; pyz = pyrazine} has a unique interpenetrating ReO₃-like lattice. The Mn²⁺ cations are high-spin (S = 5/2) and the material orders antiferromagnetically below T_N = 2.53(2) K. Using neutron powder diffraction we observed a collinear spin structure oriented along the short ac-diagonal of the monoclinic unit cell. Inelastic neutron scattering results show a magnetic excitation at 0.23 meV. The strong dispersion character of this excitation demonstrates that it is related to a low-energy spin wave. Upon warming, the magnon gradually softens and disappears at T_N, while critical scattering becomes evident by a broad quasielastic response above T_N. The energy of the magnon is consistent with the exchange parameter, J, derived from magnetic susceptibility measurements.