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> Fe > Co. As is seen in Figure 5, the metal atom displacements do follow this order. Moreover, the essentially constant Ct-N(NO) (1.93-1.98 Á) and particularly the N-N(NO) nonbonded distances (2.76-2.79 Á) provide confirmation that the nitrosyl nitrogen atom position (and by implication the metal atom position as well) is governed by minimizing the axial nonbonded interactions. We conclude that the minimum N-N nonbonded separation for an axial ligand in five-coordinate metalloporphyrins is -~2.80 Á; thus, structural adjustments must occur to prevent ••• separations significantly less than 2.80 Á.

couples have been examined for any differences in contributions of Franck-Condon and electronic terms to the rate constants. Rate constants for cross-reaction oxidations of Co(sep)2+ follow the classically predicted dependence of the Franck-Condon factor on AGab°. Far-infrared and Raman vibrational spectra of M(sep)3+/M(en)33+ ( = Co, Rh) are in very close correspondence. As a consequence, the force constants of the M-N stretching modes must also be comparable (differing by less than 10%) in the Co(sep)3+ and Co(en)33+ complexes. Strain energy calculations based on these and other published force constants are consistent with the differences in self-exchange rates and indicate upper limits on the Franck-Condon term in the rate constants consistent with significant contributions from electronic factors in both exchange reactions.