The metal-org. frameworks M2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate, MOF-74, CPO-27) (M = Mg, Mn, Fe, Co, Ni, Zn) have shown considerable promise for a no. of gas storage and sepn. applications. These have included CO2/N2, CO2/H2, O2/N2, and paraffin/olefin sepns. and the storage of H2, CH4, or acetylene. The high volumetric and gravimetric densities of coordinatively unsatd. metal cation sites, a result of the high charge and compact nature of dobdc4-, endow these materials with adsorption selectivities and capacities not displayed by other metal-org. frameworks. Here we show carbon monoxide can reversibly bind to the 5-coordinate metal cations in M2(dobdc), resulting in the first crystallog. characterized magnesium and zinc carbonyl compds. and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyls. These metal-carbonyl species have been characterized by gas adsorption isotherms, neutron powder diffraction, and IR spectroscopy. The reversible nature of CO binding to these materials suggests they may be useful for the purifn. of various carbon monoxide mixts. contg. N2, CH4, H2, or O2 impurities.