The mixed-valent {MnIII20−xMnIIx} cage complexes (x=2–6) have structures corresponding to a fragment of a cubic close-packed lattice (see picture, Mn large sphere, O small sphere). Magnetic studies show for the cage with x=2 that the ground state has a spin (S) of 19±1 in low field, which increases to about S=30 at higher fields. The cage is a “single molecule magnet” with a high energy barrier (43 K) to reorientation of the magnetization..