A centered icosahedral 12-coordinate samarium cluster formed by six bis(L-prolinato)nickel(II) ([Ni(pro)(2)]) ligands, [Sm{Ni(pro)(2)}(6)](3+), was prepared. The reaction of Sm with [Ni(pro)(2)] in a small excess (a 2-fold excess) and also in a large excess (even a 10-fold excess) of the latter produced the cluster. Therefore, this system is a self-assembly. In the cluster, each nickel atom is surrounded by six atoms: two amino nitrogens, two carboxylate oxygens which form chelate rings with the nitrogen atoms, and two carboxylate oxygen atoms which link the neighboring nickel atoms. The samarium atom is coordinated by six [Ni(pro)(2)] ligands, and the metal is in an icosahedral environment formed by 12 oxygen atoms. The icosahedral geometry is almost ideal. Crystals of [Sm{Ni(pro)(2)}(6)](ClO4)(3). 6MeOH, which were prepared from a methanol solution, immediately decompose after filtration because of loosely trapped MeOH molecules in the crystal lattice. Therefore, crystals without MeOH molecules, which must be stable, were prepared by recrystallization from acetonitrile with tetramethylammonium perchlorate (TMAP). According to the X-ray crystal analysis, the cluster is TMA[Sm{Ni(pro)(2)}(6)](ClO4)(4), cubic of space group F23, with a = 21.273(9) Angstrom, V = 9626(1) Angstrom(3), and Z = 4; R = 0.053 (R-w = 0.049) for 1296 reflections. In addition, several crystals of cluster salts that have different counteranions, i.e., tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), iodide (I-), and nitrate (NO3-), were prepared. The order of increasing ease of crystallization of the cluster salts was I-> PF6- - ClO4- > BF4- much greater than NO3-. The cluster structure is retained in alcohol and acetonitrile solutions; the UV-vis spectra of the solutions are perfectly consistent with the powder diffuse reflection spectrum. Cyclic voltammograms of [Sm{Ni(pro)(2)}(6)](3+) in acetonitrile proved that the structure of [Sm{Ni(pro)(2)}(6)](3+) is retained in the redox process and that the nickel atoms electrochemically interact with one another. Thermal analysis of the cluster salts with different counteranions was investigated. The results imply that the cluster is very stable until bis(L-prolinato)nickel(II) ligands, which form the cage structure, disintegrate and that the thermal decomposition processes of the cluster salts depend on their counteranions.