The heterogeneous reaction of crystals of the novel intermetallic subhalide Bi₁₂Rh₃Cl₂ with a solution of n-butyllithium at 70 °C led to the complete topochemical exchange of chloride ions for bismuth atoms, that is, the transformation into the isostructural metastable intermetallic superconductor Bi₁₄Rh₃. The crystals underwent the reductive pseudomorphosis almost unchanged except some fissures perpendicular to the a-axis. Detailed inspections of the transformed crystals by electron microscopy indicated no volume defects that would indicate internal chemical reactions. Thus, extensive mass transport must have occurred through the seemingly dense crystal structure. An efficient transport mechanism, based on an unusual breathing mode of the three-dimensional network formed by edge-sharing [RhBi₈] cubes and antiprisms, is proposed. The replacement of ionic interaction in the chloride by metallic bonding in the binary intermetallic compound closes the pseudo gap in the density of states at the Fermi level. As a result, the rod-packing of conducting, yet electrically isolated strands of [RhBi₈] cubes in Bi₁₂Rh ₃Cl₂ turns into the three-dimensional metal Bi ₁₄Rh₃. Breathe in: In the dense structure of Bi ₁₂Rh₃Cl₂, chloride ions are replaced by bismuth atoms in the course of a heterogeneous topochemical reaction. The vast mass transport through the crystal at only 70 °C is enabled by a breathing mode of the intermetallic framework, which opens diffusion paths by varying the inclination ω (see picture). The complete exchange yields the metastable superconductor Bi₁₂Rh₃Bi₂.