Catalysis of a symmetry restricted reaction by transition metal complexes. The importance of the ligand

The valence isomerization of quadricyclene to norbornadiene is catalyzed by certain transition metal complexes of phthalocyanine and tetraphenyl porphyrin. To explore the dependence of catalytic activity on the number of d-electrons in the metal ion, complexes having a number of d-electrons from 5 to 10 were studied. Most of the d6, d7 and d8 ions were active whereas the d9 and d10 ions were inactive. The influence of the stereochemistry of the ligand was studied using cobalt (II) complexes with certain quadridentate Schiff bases whose stereochemistry could be changed by small modifications in structure. Square planarity of the complex appeared to be a condition for catalytic activity. The catalytic reaction could be inhibited by the addition of pyridine bases. 4-Picoline was more effective than pyridine, but 2-picoline did not poison the reaction at all. These results could be explained within the framework of existing theories dealing with this form of catalysis.