Despite the critical functions of divalent metal ions (M(2+)s) in association with duplex DNA, fundamental and general interactions of M(2+)s with spherical nucleic acids (SNAs) composed of single-stranded DNA have rarely been investigated. We have explored that the coordinative nature of the M(2+)-SNA binding mediates the temperature- and base composition-dependent reversible assemblies of SNAs even without the need of complementary counterparts for duplex-interconnection, additional monovalent metal ions for charge screening, or pre-designed sequences for any non-Watson-Crick base-pairing, all of which are essential for the conventional assembly of SNAs. Cu(2+) has been identified to maximize the reversible assembly properties in relation to this M(2+)-mediated DNA bond, and has been further qualitatively and quantitatively investigated in detail as a model system.