A flexible 3D porous magnet [KCo7(OH)(3)(ip)(6)(H2O)(4)]center dot 12H(2)O based on unprecedented trigonal-prismatic heptanuclear Co-II clusters (see figure) is reported to undergo a reversible single-crystal-to-single-crystal transformation on the desorption/adsorption of guest molecules, accompanied by reversible magnetic property changes. Ibis might be applied for the sensing of coordinative, paramagnetic, small molecules such as 02 and NO.