Metal sites play a central role in the vast majority of molecular recognition processes involving biological and synthetic extended systems due to their ability to impart highly selective and specific molecular transformations, transport, and storage.1-4 Although extensive research efforts worldwide have been devoted to studying the result of chemical reactions at metal centers, definitive structural characterization of open metal (OM) sites, that are coordinatively unsaturated, by single crystal X-ray diffraction have been largely absent. The inherent difficulty in stabilizing such entities in molecular scaffolds arises from their reactive nature and from the lack of structural rigidity around the sites they occupy, which often leads to their aggregation and severe distortion of their local structure features that preclude their characterization as OM sites. In this report, we present a design strategy based on knowledge gained from modular chemistry,5-14 for producing high concentrations of covalently bound and accessible OM sites into crystalline porous materials: An organic adamantane tetrahedral cluster is copolymerized with an inorganic square cluster to yield a porous metal-organic framework (MOF) having a 3-D channel system filled with water guests. The rigid nature of the MOF architecture allows for complete thermal removal of guests including terminal water ligands, originally bound to copper sites, of the square pyramidal clusters to yield a stable framework with periodic arrays of open copper sites. These observations are supported by single-crystal X-ray structures, gas sorption isotherms, elemental microanalyses, exceptional bond lengths, and magnetic coupling