Treatment of aluminum nitrate with an organic nitroso-containing compound yields the “flat”, tridecameric nanocluster Al13(μ3-OH)6(μ2-OH)18(H2O)24(NO3)15 (Al13) in good yield on a preparative scale under ambient conditions. Synthetic procedures yielding two different single-crystal forms of the Al13 cation with two varying counterion compositions are described.