For applications such as gas separations, heterogeneous catalysis, and sensing, porous adsorbents offer an energy-efficient alternative to established technologies. In particular, metal-organic frameworks (MOFs) bearing coordinatively unsaturated metal sites can bind gas molecules with selectivities one or two orders of magnitude larger than traditional adsorbent materials that possess non-specific physisorption interactions. Through judicious choice of the metal ion and linker, one can tailor for example strong selective gas binding under a plethora of conditions extending into industrial gas separations, catalysis, and gas storage. Unsaturated metal sites characteristic of some adsorbents-including those in MOFs and zeolites-are electronically Lewis acidic in nature with minimal π-backbonding capabilities. In contrast, π-basic metal sites afford the possibility of strongly adsorbing many substrates. Lewis π-basic sites in MOFs are less common and act either by accepting electrons in free orbitals or donating electrons into adsorbate accessible π* orbitals, which are referred to as back bonding and forward donation, respectively.