A detailed kinetic and mechanistic analysis of the classical "brown-ring" reaction of [Fe(H2O)(6)](2+) with NO was performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic parameters for the "on" and "off" reactions at 25 degreesC were found to be k(on) = 1.42 x 10(6) M-1 s(-1), DeltaH(on)(double dagger) = 37.1 +/-0.5 kJ mol (-1), DeltaS(on)(double dagger) = -3 +/- 2 J K-1 mol(-1), DeltaV(on)(double dagger) = + 6.1 +/- 0.4 cm(3) mol(-1), and k(off) = 3240 +/- 750 s(-1), Delta(off)(double dagger) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(off)(double dagger) = -15 +/-5 J K-1 mol-1, DeltaV(off)(double dagger) = +1.3 +/- 0.2 cm(3) mol(-1). These parameters suggest that both reactions follow an interchange dissociative (I-d) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H2O)(6)](2+). In addition, Mossbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H2O)(5)(NO)](2+). The Mossbauer and EPR parameters closely resemble those of the {FeNO}(7) units in any of the other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of high-spin Fe-III antiferromagnetically coupled to NO- (S = 1) yielding the observed spin quartet ground state (S= 3/2), i.e., [Fe-III(H2O)(5)(NO-)](2+), and not [Fe-I(H(2)0)(5)(N0(-))](2+) as usually quoted in undergraduate text books.