With this award the Inorganic, Bioinorganic, and Organometallic Program continues its support of research by Dr. Daryle H. Busch of the Chemistry Department, University of Kansas, concerning the binding of O2 (dioxygen) to transition metal complexes. The goal of the research is to control the conformation of iron(II) or cobalt(II) complexes and thus enable reversible dioxygen binding at a single site or at two or more correlated sites. Tetradentate and pentadentate ligands which impose structural constraints will be used to iteratively synthesize complexes which may have the desired properties. Specifically, the ligands tested will belong to one of six classes: porphyrins, cyclidenes, alpha-dioximes, or one of three types of macromolecules from the Schiff base family. Measurements of equilibrium constants and rate constants for oxygen binding and autoxidation will be used to assess the affinity of the site for oxygen and determine the mechanism of oxidation by dioxygen. %%% Transition metal complexes which can bind and carry atmospheric oxygen will be synthesized. The shape of various groups attached to the metal will be varied in order to find ways to control the ease with which the oxygen molecule is attached and released by the metal and to prevent the oxygen from reacting with the metal compounds. The chemistry of these compounds is relevant to understanding the binding and transport of oxygen in respiring organisms and to the potential development of many useful devices, such as fuel cells and sensors, and synthetic blood.