Here we show that dioxygen binding to vacant metal ion sites proceeds at rapid rates, with very low enthalpic barriers but negative activation entropies; the rates are relatively insensitive to the electronic structure of the other ligands bound to the metal. While these properties are shared by vacant sites exposed to noncoordinating solvents and by those having sites protected from solvent, the latter react substantially more rapidly. The oxygenation of cobalt(II) complexes with pentadentate Schiff base ligands dissolved in the noncoordinating solvent acetone has been studied. For nonbridged complexes, reversible formation of 1:1 adducts with O(2) was observed only at low temperatures (from -75 to -40 degrees C for acetylacetone derivatives and from -75 to -20 degrees C for salicylaldehyde derivatives), whereas the oxygenation of the corresponding lacunar species is reversible at room temperature. The dioxygen affinity of the salicylaldehyde derivative (CoSalMeDPT) (0.024 Torr(-)(1) at -39 degrees C) is significantly smaller than that of the analogous unbridged and p-xylene-bridged acetylacetone derivatives (>5 Torr(-)(1) and 1.8 Torr(-)(1) at -40 degrees C, respectively), presumably because of the lower electron-donating ability of the ligand. The dynamics of O(2) binding to a vacant cobalt(II) coordination site proved to be fast (on the order of 10(6) M(-)(1) s(-)(1) for unbridged complexes and up to 10(8) M(-)(1) s(-)(1) for the bridged ones), due to an extremely low activation barrier (1-3 kcal/mol for both unbridged complexes). The differences in the electronic structures of the ligands is reflected primarily in their O(2) dissociation rates, while steric effects produce significant differences in O(2) binding rates.