Axial Ligation Constants of Iron(II) and Cobalt(II) "Capped" Porphyrins

Article abstract of DOI:10.1021/ja00526a022

Equilibrium constants for addition of ligands to cobalt(II) and iron(II) complexes of the "capped" and "homologous capped" porphyrins are reported.For the equilibrium Fe^II(cap) + B <*> Fe^II(Cap)(B) (K^B), a plot of log K^B vs. the pK_a of the conjugate acid of the ligand shows a linear relationship among structurally similar base ligands. ?-Bonding ligands form more stable complexes than predicted by their pK_a values.Sulfur base binding to Co^II(Cap) shows a relative binding order for S-donor ligands of thiolates >> thioethers > mercaptans.Unlike the Fe(Cap), which can bind only one axial base ligand, Fe(HmCap) can bind two small ligands axially in the usual manner.Intermediate-size ligands, such as 1-methylimidazole, however, weakly coordinate a second base leading to an unusual six-coordinate complex with an intermediate electronic spin of S = 1.