The quest for the single-molecular magnets (SMMs) based on mononuclear transition-metal complexes is focused on the low-coordinate species. No transition-metal complex with a coordination number of eight has been shown to exhibit SMM properties. Here the magnetic studies have been carried out for a mononuclear, eight-coordinate cobalt(II)-12-crown-4 (12C4) complex [Co-II(12C4)(2)](I-3)(2)(12C4) (1) with a large axial zero-field splitting. Magnetic measurements show field-induced, slow magnetic relaxation under an applied field of 500 Oe at low temperature. The magnetic relaxation time tau was fitted by the Arrhenius model to afford an energy barrier of U-eff = 17.0 cm(-1) and a preexponential factor of tau(0) = 1.5 X 10(-6) s. The work here presents the first example of the eight-coordinate, mononudear, 3d metal complex exhibiting the slow magnetic relaxation.