We have recently reported that aquo and thioether complexes of the ferric cytochrome c heme peptide N-acetylmicroperoxidase-8 (Fe-III-1) exhibit greater low-spin character than do the corresponding complexes of a synthetic, water-soluble, monohistidine-ligated heme peptide (Fe-III-2; Cowley, A. B.; Lukat-Rodgers, G. S.; Rodgers, K. R.; Benson, D. R. Biochemistry 2004, 43, 1656-1666). Herein we report results of studies showing that weak-field ligands bearing a full (fluoride, chloride, hydroxide) or partial (phenoxide, thiocyanate) negative charge on the coordinating atom trigger dissociation of the axial His ligand in Fe-III-2 but not in Fe-III-1. We attribute the greater sensitivity of His ligation in Fe-III-1 to weak-field anionic ligands than to weak-field neutral ligands to the following phenomena: (1) anionic ligands pull Fe-III further from the mean plane of a porphyrin than do neutral ligands, which will have the effect of straining the His-Fe bond in Fe-III-2, and (2) heme in Fe-III-2 is likely to undergo a modest doming distortion following anion binding that will render the His-ligated side of the porphyrin concave, thereby increasing porphyrin/ligand steric interactions. We propose that ruffling of the heme in Fe-III-1 is an important factor contributing to its ability to resist His dissociation by weak-field anions. First, ruffling should allow His to more closely approach the porphyrin than is possible in Fe-III-2, thereby reducing bond strain following anion binding. Second, the ruffling deformation in Fe-III-1, which is enforced by the double covalent heme-peptide linkage, will almost certainly prevent significant porphyrin doming.