FTIR Investigation of the Intermediates Formed in the Reaction of Nitroprusside and Thiolates

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We have studied the outer-sphere reduction of [Fe(CN)5(NO)]2- by several reagents including dithionite and have for the first time measured the IR spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- in aqueous media. The spectra of [Fe(CN)5(NO)]3- and [Fe(CN)4(NO)]2- are consistent with bent six-coordinate {MNO}7 and linear five-coordinate {MNO}7 species, respectively. We have measured the UV-visible and IR spectra that evolve after [Fe(CN)5(NO)]2- is reacted with thiolate. These spectra permit us to assign the molecular structure of the so-called "red product" as [Fe(CN)5(eta1-N-RSNO)]3-. We have followed the decomposition of the [Fe(CN)5(eta1-N-RSNO)]3- by IR. Importantly, there is a 1:1 correspondence between the disappearance of the [Fe(CN)5(eta1-N-RSNO)]3- and the formation of [Fe(CN)5(NO)]3-. Thus, we conclude under the conditions of this study, reduction of [Fe(CN)5(NO)]2- by thiolate takes place via a (dark) inner-sphere mechanism that yields [Fe(CN)5(NO)]3- via homolytic N-S bond cleavage.