Bis(1,1,1-trifluoro-5,5-dimethyl-5-methoxy-acetylacetotato)copper(II) was prepared in two polymorphic modifications. The orthorhombic alpha-form is stable and densely packed, with four trans and four cis square bischelate building blocks per unit cell. These are connected through additional coordination bonds to form a dense polymer network. For the trigonal beta-form, the square bischelate complex units are present exclusively as the trans isomers. The distinctive assembly of these units results in a lattice with an open pore volume of about 17% that is accessible to a wide range of guests. The compound has a remarkably strong affinity for the porous beta-form as evident from the efficient alpha-to-beta conversion on contact not only with liquid guests but also with organic vapors at pressures well below the saturation pressure. Although the open beta-form is metastable, it has a remarkable kinetic stability, most likely because of the trans-to-cis isomerization that must accompany the beta-to-alpha transformation. Many sorbents play a dual role as stabilizing guest and as catalyst promoting the alpha-to-beta or beta-to-alpha conversion. Because of its versatile sorption properties and relative robustness. the beta-form of the complex can be classified as a novel organic zeolite mimic.