The ligand exchange in (n-Bu4N)2OsIVCl6 (n-Bu4N = tetra-n-butylammonium) leads to the formation of the osmium(IV) heptacyanide, the first fully inorganic homoleptic complex of heptacoordinated osmium. The single-crystal X-ray diffraction (SC-XRD) study reveals the pentagonal bipyramidal molecular structure of the [Os(CN)7]3− anion. The latter being a diamagnetic analogue of the highly anisotropic paramagnetic synthon, [ReIV(CN)7]3− can be used for the synthesis of the model heterometallic coordination compounds for the detailed study and simulation of the magnetic properties of the low-dimensional molecular nanomagnets involving 5d metal heptacyanides.