A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H2O)(2)]ClO4 (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/-0.06 at 25 degreesC). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degreesC and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M-1 s(-1), DeltaH(f(I))(#) = 62 +/- 3 kJ mol(-1), DeltaS(f(I))(#) = -30 +/- 10 J K-1 mol(-1), and DeltaV(f(I))(#) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an 1, mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M-1 s(-1) at 25 degreesC, respectively. The more labile aqua-hydroxo complex is suggested to follow an Id or D substitution mechanism on the basis of the reported kinetic data. At pH greater than or equal to4.5, the second substitution step also can be monitored (pH 5.5 and 25 degreesC: k(f(II)) = 21.1 +/- 0.5 M-1 s(-1), DeltaH(f(II))(#) = 60 +/- 2 kJ mol(-1), DeltaS(f(II))(#) = -19 +/- 6 J K-1 mol(-1), and DeltaV(f(II))(#) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I-d or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.