The anions and cations of the complex [Fe(cp)(2)][FeBr4] (cp = h(5)-cylopentadiene) behave as isolated non-interacting Heisenberg and Ising layers respectively. The S = 1/2 cations order at similar to 2.5 K while the S = 5/2 anions order at similar to 12.5 K. A variety of experimental data confirm independent 3D antiferromagnetic ground states for the component ions as opposed to the usual expectation of ferrimagnetism resulting from the collinear antiferromagnetic interaction of cation and anion sublattices of highly different spin in e.g., a simple two magnetic sublattice system.