In the reaction of [Fe(CN)
5NO
]3- with O
2 (4:1 stoichiometry), the nitroprusside ion [Fe(CN)
5NO
]2- (NP) was quantitatively produced. The rate law was -1/4d[Fe(CN)
5NO
3-]/d
t =
k2[Fe(CN)
5NO
3-][O
2], with
k2 = (3.5 ± 0.2) × 10
5 M
-1 s
-1 at 25.0
C. The rate was insensitive to pH (range 9-11) and ionic strength (
I = 0.1-1 M). Excess cyanide was used for minimizing the production of [Fe(CN)
4NO
]2-, which appeared to be much less reactive. Addition of O
2 to the NO
group is proposed to generate peroxynitrite bound to Fe(III), which reacts rapidly with [Fe(CN)
5NO
]
3- to yield [Fe(CN)
5NO
2]
3-. Reaction between the latter radical ions leads to the final product, NP. Comparison with the autoxidation rates for other NO
complexes reveals that the rate constants decrease with an increase in
ENO+/NO, the redox potentials of the bound NO
+/NO
couples. Six-coordination appears to be necessary for attaining reasonable electrophilic rates for the NO
ligand, as with other heme and non-heme complexes.