Treatment of aluminum nitrate with an organic nitroso-containing compound yields the "flat", tridecameric nanocluster Al-13(mu(3)-OH)(6)(mu(2)-OH)(18)(H2O)(24)(NO3)(15) (Al-13) in good yield on a preparative scale under ambient conditions. Synthetic procedures yielding two different single-crystal forms of the Al-13 cation with two varying counterion compositions are described.