This ligand is reversibly bound by solvated derivatives of a complex derived from cobalt and the salen ligand known as salcomine. This bridging ligand links together two copper(I) (“copper-one”) centers in a superfamily of proteins that contains hexamerins. The reversible binding of this molecule in a side-on fashion induces a color change from yellow to orange in Vaska’s complex. In one complex, binding of this molecule above the plane draws the metal center away from a proximal histidine residue and induces diamagnetism. This molecule, whose binding curve follows a sigmoidal (10[1])shape on the original (-5[1])Hill plot, has a binding affinity over 200 times weaker than that of carbon monoxide for one complex. For 10 points, name this molecule transported by hemoglobin. (10[1])■END■ (10[6])