We have previously shown (Inorg. Chim. Acta 1984, 91, 95-102) that the formation constants of all zinc(I1) tetraphenylporphyrins obey a common linear free energy relationship with the sum of the Hammett u constants of the substituents, irrespective of whether the tetraphenylporphyrin is symmetrically or unsymmetrically substituted. In contrast, iron(II1) tetraphenylporphyrins exhibit a more complex behavior. For the general reaction, tetra-para-substituted TPP complexes obey 82 PFeCl + 2N-MeIm . CHCIJ, 25 oC ' PFe(N-MeIm),+CIone linear free energy relationship, with p = -0.39, while tetra-meta-substituted and unsymmetrically multiply para-substituted TPP complexes, [(p-X),(p-Y)4,TPP] FeC1, obey a different linear free energy relationship with p = -0.18. Monosubstituted TPP complexes, [ (mor p-X)lTPP] FeC1, are insensitive to the electronic effect of the substituent. These results have led to a reevaluation of the iron(II1) porphyrin-axial ligand complex formation system, in which both monoand tetra-para-substituted TPPs are now viewed as behaving anomalously. The reason for this anomalous behavior is believed to be nonadditivity of the substituent effects, leading eventually to the direct conjugation of the fourth phenyl ring in the product, [@-X),TPP]Fe(N-MeIm)2tC1-, in order to remove the formal positive charge on Fe(II1). The unsymmetrically multiply para-substituted TPP derivatives are also believed to engage in direct conjugation between phenyl and porphyrin ring, but by choosing to utilize the least willing substituent in this conjugation. The variation in energy of the Q, electronic with the ratio of intensity of the Q, and QV transitions for the bis(N-methylimidazole) adducts is in accord with an important role for direct conjugation in these low-spin complexes. The degree and mode of transmission of electronic effects to and from various points on the porphyrin ring through the u and a orbitals of the four porphyrin nitrogens to the central metal have long been an interest of those who have investigated the physical properties and chemical reactions of m e t a l l ~ p o r p h y r i n s . ~ ~ ~ ( 1 ) ReciDient. NIH Research Career Develomt Award. 1976-1 98 1. (2j Falk: J. E: Porphyrins and Meta1loporph;rins; else vier^: New York, (3) Caughey, W. S.; Deal, R. M.; McLees, B. D.; Alben, J . 0. J . Am. 1964; pp 28, 42, 69, 93. Chem. SOC. 1962, 84, 1735-1736. (4) Caughey, W. S.; Fjuimoto, W. Y.; Johnson, B. P. Biochemistry 1966, ( 5 ) McLees. B. D.: Caughev. W. S. Biochemistrv 1968. 7 . 642-652. 5, 3830-3843. (6) Baker, E. W.; Storm:C.'B.; McGrew, G. T.; Corwin,'A: H. Bioinorg. (7) Higginbotham, E.; Hambright, P. Inorg. Nucl. Chem. Lett. 1972, 8, Chem. 1973, 3, 49-60. 747-150 . . (8) Worthington, P.; Hambright, P.; Williams, R. F. X.; Feldman, M. R.; Smith, K. M.; Langry, K. C . Inorg. Nucl. Chem. Lett. 1980, 46, 441-447. (9) Worthington, P.; Hambright, P. Williams, R. F. X.; Reid, J.; Burnham, C.; Shamim, A,; Turay, J.; Bell, D. M.; Kirkland, R.; Little, R. G.; DattaGupta, N.; Eisner, U. J . Inorg. Biorhem. 1980, 12, 281-291. (10) Williams, G. N.; Hambright, P. Inorg. Chem. 1978, 17, 2687-2688. (1 1) Kirksey, C. H.; Hambright, P.; Storm, C. B. lnorg. Chem. 1969, 8, (12) Walker, F. A.; Hui, E.; Walker, J. M. J . Am. Chem. Soc. 1975, 97, (13) Walker, F. A.; Beroiz, D.; Kadish, K. M. J . Am. Chem. SOC. 1976, (14) Walker, F. A,; Lo, M.-W.; Ree, M. T. J . Am. Chem. Soc. 1976, 98, (15) Satterlee, J. D.; LaMar, G. N.; Frye, J. S. J . Am. Chem. SOC. 1976, (16) Vogel, G. C.; Beckmann, B. A. Inorg. Chem. 1976, 15 , 483-484. (17) McDermott, G. A.; Walker, F. A. Inorg. Chim. Acta 1984, 91, (18) Jones, R. D.; Summerville, D. A.; Basolo, F. J . Am. Chem. SOC. 1978, 2 14 1-2 144.