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 (-5[1])draws the metal center away from a proximal histidine residue and induces diamagnetism. (10[1])This molecule, whose binding curve follows a sigmoidal (10[1])shape on the original Hill (-5[1])plot, (10[1])has a binding affinity over (10[1])200 times weaker than that of carbon monoxide for one complex. For 10 points, name this molecule transported by hemoglobin. (10[1])■END■ (10[1]0[2])