Contrary to expectations based on the spectrochemical series, H 2 O is found to be a significantly weaker field ligand than OH -in the magnetochemical series ranking of ligand field strengths based on the spin states of iron(III) tetraphenylporphyrin complexes. The preparation and characterization of the [Fe(H 2 O)-(TPP)] + ion and the spectroscopic identification of Fe(OH)(TPP) have made this assessment possible. These two species were previously thought to be unattainable because of the facile formation of the well-known µ-oxo dimer, (TPP)Fe-O-Fe(TPP). However, the special characteristics of single equivalents of water under high acidity, relevant to metalloenzyme active sites and superacidity, make them accessible in benzene solution. Their 1 H NMR -pyrrole chemical shifts at -43 and +82 ppm indicate admixed-intermediate S ) 3 / 2 , 5 / 2 and high S ) 5 / 2 spin states for the aqua and hydroxo species, respectively. The X-ray crystal structure of the aqua complex has been determined for [Fe(H 2 O)(TPP)][CB 11 H 6 Cl 6 ] and is consistent with the high degree of S ) 3 / 2 character indicated by the NMR measurement, Mössbauer spectroscopy (∆E q ) 3.24 mm‚s -1 ), and magnetic susceptibility (µ eff ) 4.1 µ B ). The anhydrous precursor to these species is the "nearly bare" iron(III) porphyrin complex Fe(CB 11 H 6 Br 6 )(TPP). Judged by its magnetic parameters (δ pyrrole ) -62 ppm, ∆E q ) 3.68 mm‚s -1 , µ eff ) 4.0 µ B ) it attains the long sought essentially "pure" S ) 3 / 2 spin state. The magnetochemical ranking of ligand field strengths in five-coordinate high-spin and admixed-intermediate-spin iron(III) porphyrins is useful because it more closely reflects the intuitive field strengths of crystal field theory than does the usual spectrochemical ranking, which is controlled largely by π effects in octahedral low-spin d π 6 complexes.