The complex anion bis(azido)(tetraphenylporphinato)ferrate(III) has been synthesized and characterized by variable-temperature X-ray structure determinations, powder and single-crystal EPR, IR, and Mossbauer spectroscopy, and magnetic susceptibility measurements. The synthesis utilizes 18-crown-6 to solubilize sodium azide in the synthetic procedure. All physical data for [Na(18C6)(H2O)(2)][Fe(TPP)(N-3)(2)]. 2C(6)H(5)Cl are consistent with a thermal spin-equilibrium system: low spin (S = 1/2) reversible arrow high spin (S = 5/2). Structure determinations at 130 and 293 K show equatorial and axial Fe-N bond elongation at 293 K. Fe-N-p = 1.9991(11) Angstrom, Fe-N-az = 1.9734(14) Angstrom at 130 K, and Fe-N-p = 2.010(4) Angstrom, Fe-N-az = 1.998(2) Angstrom at 293 K. The EPR g values for a powder sample at 4.2 K are 1.81, 2.18, and 2.70. A fit of the powder EPR spectrum at 4.2 K with a crystal field model that allows quartet and sextet admixtures suggests that the first sextet state is similar to 655 cm(-1) above the ground doublet. Single-crystal EPR data indicate that the largest g value occurs at an angle of 56 degrees from the porphyrin normal and at 35 and 81 degrees from the Fe-N-p vectors. The asymmetric azide IR absorption bands at 2014 and 2036 cm(-1) can be assigned to low- and high-spin species, respectively, and display temperature-dependent intensities. The Mossbauer experiments reveal a gradual decrease in the quadrupole splitting as the temperature increases from 140 to 300 K. The magnetic susceptibility measurements show a gradual increase of mu(eff) with temperature. Crystal data for [Na(18C6)(H2O)(2)][Fe(TPP)(N-3)(2)]. 2C(6)H(5)Cl (130 K): a = 11.417(2) Angstrom, b = 12.371(4) Angstrom, c = 12.628(2) Angstrom, alpha = 64.30(2)degrees, beta = 77.18(3)degrees, gamma = 77.67(2)degrees, triclinic, space group P (1) over bar, Z = 1. Crystal data (293 K): a = 11.7652(12) Angstrom, b = 12.6488(6) Angstrom, c = 12.8608(13) Angstrom, alpha = 62.02(2)degrees, beta = 75.996(7)degrees, gamma = 75.465(9)degrees.