Dynamics of CO binding to lacunar iron(II) cyclidene complexes

The kinetics of carbon monoxide binding and dissociation have been studied for a series of lacunar iron(II) cyclidene complexes to elucidate the dependence of dynamic parameters on the various structural features of these versatile compounds. Ligand substituents have large effects on the binding and dissociative rate constants, and remarkably, four distinctly different steric effects have been observed. (1) Changing cavity size alters the rate of CO binding by as much as 4 orders of magnitude, presumably by constraining access to the metal ion. (2) Decreases in cavity size also can increase the rate of CO dissociation by a factor of 10 or so. (3) Placing bulky groups in the path CO must follow to enter the cavity decreases the rate of binding because of steric effect (1), but these same obstructions may also decrease the rate of dissociation by blocking the escape path and, possibly, fostering geminate recombination. (4) Proximal ligand strain both decreases the rate of binding and increases the rate of CO dissociation. In contrast, changes in the iron(III)/(II) redox potential, which accompany ligand substitutions, were found to have only a small impact on CO binding kinetics. The effects on the rate constants of the basicity of the axial base and of solvent polarity were also investigated.