The complex tris(2-(aminomethyl)pyridine)iron(II), Fe(AMP)32+, is the first iron(I1) system in which the high-spin form is labile that exhibits observable magnetic isomerism. Stable solutions of this complex are obtained in acetonitrile/water, methanol/water, and water solvent systems provided that there is an excess of free 2-(aminomethyl)pyridine, that the solution is at pH 8-9, and that no molecular oxygen is present. The interpretation of the solution magnetic moment measurements by the Evans method, EPR, and UV-visible absorption spectra as a function of temperature allows us to describe the spin equilibrium as Fe(AMP)32+ (t2;, 'AlE) s Fe(AMP),2+ (t 2: e: , 5A,). The 'A, ground state has g = 2.01 and is separated from the 5E state by d = 850 cm-'. Both 5E and 'A, result from the splitting of the 2T2E octahedral level, and d measures the degree of axial distortion. The separation between 'A, and 'E at 300 K is 1100 cm-'. The thermodynamic parameters for this equilibrium are AH = 5.1 A 0.4 kcal/mol and LLS = 17 f 2 cal/(K mol) in the acetonitrile solvent system. Lower values for these parameters were found in the more polar solvents. Visible and ultraviolet absorption spectra show two intense charge-transfer bands and two weak shoulders, probably associated with d-d transitions. Ligand field parameters for these complexes have been estimated on the basis of the spectral assignments as Ahs = 13 700 cm-I, AIS = 18 700 cm-l, B (Racah parameter) = 750-800 cm-l, and, T (the mean electron pairing energy) = 16 100 cm-I.