Electron spin echo envelope modulation (ESEEM) and extended X-ray absorption fine structure (EXAFS) spectroscopic studies of oxygenated cobalt (oxyCo) [tetraphenylporphyrin(TPP)][1-methylimidazole (1-MeIm)], an active site model of oxyCo-substituted globins (functional and EPR-active [S = 1/2] analogues of oxygen carrying hemoproteins), are carried out in order to examine the correlation of oxygen affinity with electron-nuclear coupling parameters and metal-ligand bond lengths. ESEEM demonstrates that the magnitude of the electron-nuclear hyperfine and nuclear quadrupole couplings to the directly-coordinated N-14 Of 1-MeIm decrease (A(iso) from 3.54 to 3.04 MHz; e(2)qQ, from 2.39 to 2.08 MHz) as the solvent composition is varied from 0 to 50% (v/v) dichloromethane in toluene. For oxyCo[(o-R)(1)TPP][1-MeIm] (where R = -H, -NHCOC(CH3)(3), -NHCOCH3, or -NHCONHC6H5, an ortho substituent on one of the four meso phenyls of TPP), couplings to the axial nitrogen decrease (A(iso) from 3.54 to 3.07 MHz; e(2)qQ, from 2.39 to 2.09 MHz) with increased electron-withdrawing strength of R, i.e., with increased acidity of the amide proton of R that may interact with the bound dioxygen. EXAFS measurements, and analysis using ab initio EXAFS codes and global mapping, find that the cobalt-axial nitrogen (N-ax) bond of oxyCoTPP-1-MeIm shortens by 0.18 +/- 0.06 Angstrom when the solvent is changed from 100% toluene (Co-N-ax = 2.12 Angstrom) to 50% toluene/50% dichloromethane (Co-N-ax = 1.94 Angstrom). The average cobalt-equatorial nitrogens (1.94-1.96 Angstrom) and cobalt-oxygen (1.95-1.98 Angstrom) distances are unchanged within the error. Similar results were obtained when oxyCoTPP-1-MeIm was compared (in 100% toluene) with its (o-NHCONHC6H5)(1)TPP counterpart, where the cobalt-ligand bond lengths are indistinguishable from those of oxyCoTPP-1-MeIm in 50% toulene/50% dichloromethane. increasing the polarity of the solvent and of the vicinity of the bound dioxygen increases oxygen affinity of the metal due to an increase in the ionicity of the cobalt-dioxygen bond that is manifested in reduction in electron-nuclear couplings to the axial nitrogen [Lee et al. Biochemistry 1994, 33, 7609] and shortening of the cobalt-axial nitrogen bond. These ESEEM and EXAFS characterizations of metal-ligand interactions demonstrate the correlation of electron-nuclear coupling and metal-Ligand bond lengths with oxygen affinity of hemoprotein model complexes.