The Mossbauer effect associated with (129)I, (125)Te, and (57)Fe has been applied to investigate structural properties of the axial ligands in ferric-high spin hematoheme-I, hematoheme-Te, and hematoheme-histidine-iodide. The magnitude and sign of the (129)I quadrupole coupling constant (e(2)qQ) and the isomer shift, as deduced from the Mossbauer effect spectra, are consistent with an axial, s-p hybridized bond of overwhelming sigma-character. Identical coupling constant (e(2)qQ((127)I) = -1826 +/- 3 MHz) were measured for both heme-I and hematoheme-histidine-iodide. Implications of this result to the nature of ferric-high spin heme-histidine complexing are discussed. A stable heme (125)Te species formed in the electron-capture decay of heme-(125)I is observed, and structural properties of this singly bonded telluride ligand are discussed.