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Sigma- and pi- Bonding Modes of Pyridine and Imidazole Type Ligands in the Transition States of Their Reactions with [Co(III)(protoporphyrin IX dimethyl Ester)(MeO)(MeOH)] in Methanol

The rates of replacements of MeOH by pyridine (py) and imidazole (imH) type ligands L (=4CN-py, 3CN-py, 3Cl-py, 4Cl-py, py, 4Et-py, 4Me-py, 4NH2-py, 4(Me)2N-py ; 4, 5(Cl)2-imH, 4, 5(CN)2-imH, Bz-imH, imH, 1Et-im, 1Me-im, 2Me-imH, 2Et-imH) in [(CoIIIP(MeO)(MeOH)] (P = protoporphyrin IX dimethyl ester) were measured by stopped-flow techniques. The methoxide ligand is firmly held while the MeOH ligand is labile and is replaced by L in a dissociative (D) mechanism. The MeO- "orienting" ligand, an excellent electron donor, favors entry of the least basic L, not the most basic as usually observed. The plot of ln kobs vs pKa of pyridine and its derivatives exhibits a minimum rate at pKa of about 5. The "V" diagram is explained as being due to a change in the electronic structure of the transition state, from predominantly bonding (descending branch) to the predominantly bonding (ascending branch). Imidazoles show similar trends, but their rates level off for pKa values above 7. The free energy of activation for the reaction between [CoIIIP(MeO)] and L is more sensitive to the change in the strength of the bond than to that of the bond.

(4-methylimidazole)pentaamminecobalt(iii) trichloride dihydrate. Iron(iii) porphyrin microperoxidase-8. Dissociative interchange mechanism. Substitution-reactions. Crystal-structure. Cytochromes-b. Coordination. Complexes. Aquahydroxocobinamide. Hemoproteins.

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