Sulfur base ligation to iron(II) and cobalt(II) porphyrins.

Equilibrium constants for the binding of sulfur bases to cobalt(II) porphyrins were measured in toluene solution by a spectrophotometric method. The order of decreasing binding strength of sulfur ligands to cobalt(II) porphyrins was found to be mercaptide ions >> thioethers > mercaptans. It is suggested that a similar stability order of these sulfur ligands should exist towards iron(II) porphyrins, but formation constants could be obtained only for the mercaptide ions.