Encapsulation by synthetic hosts can transform the spin states of square planar Ni(II)(acen) and Co(II)(tap) complexes. Upon encapsulation, the red Ni(II) diamagnetic state was converted into a green paramagnetic state, whereas the Co(II) low spin (S = 1/2) state was changed into a coupled (S = 1/2 and S = 3/2) state. The host cages are noninnocent and host-guest interactions within the confined cavity influence the resultant properties of the enclathrated metal complexes.