Described are the preparation and functional characterization of nanocrystalline and/or amorphous thin films comprising of neutral "molecular squares" of the form [Re(CO)(3)(Cl)(mu-L)](4) (L = difunctional imine or azine ligand). The films are strongly adherent, stable in aqueous media, and characterized by comparatively few pinhole defects. Electrochemical transport experiments show that the materials are exceptionally porous with respect to sufficiently small solution-phase permeants but blocking toward larger permeants. Related thin-film experiments based on monometallic "corner" materials indicate efficient exclusion of all candidate permeant molecules evaluated. For the title materials, these experiments, together with additional electrochemical probe experiments, indicate that (1) membranelike permeation via pores or tunnels of about nanometer diameter is the primary mode of transport of molecular and ionic species through thin films and (2) the transport-relevant pore or tunnel diameter is defined by the cavity dimensions for the component molecular square. The crystal structure of a single isomer of [Re(CO)(3)(Cl)(mu-4,4'-bipyridine)](4) is also reported. A packing view down the c axis of the tetragonal unit cell shows that the molecules, which are significantly puckered in the crystalline state, are arranged with cavities aligned to generate infinite zeolite-like channels.