A study of mercury(II)-catalysed substitution of cyanide in hexacyanoferrate(II) by 2-methylpyrazine: effects of inhibitor on the catalysed reaction Neetu Bansal Received: 14 April 2009 / Accepted: 18 June 2009 / Published online: 11 July 2009 Ó Springer Science+Business Media B.V. 2009 Abstract The kinetics and mechanism of the mer- cury(II)-catalysed ligand substitution reaction between potassium hexacyanoferrate(II) and 2-methylpyrazine (2- Mepz), in potassium hydrogen phthalate buffer of pH 3.50 ± 0.02 at 25.0 ± 0.1 °C and constant ionic strength of 0.1 M, were studied spectrophotometrically by mea- suring the absorbance of the yellow-coloured product at 447 nm (k max ). The reaction was studied under pseudo- first-order conditions using excess of 2-Mepz. The reaction was found to obey first-order dependence in concentration of both [Fe(CN) 6 ] 4- and 2-Mepz. At higher concentrations of 2-Mepz, the effect of [2-Mepz] levels off. A complex behaviour is observed on varying [Hg 2? ]. The effect of pH on the reaction rate was also analysed. The effects of dielectric constant and water content of the reaction med- ium have been interpreted in terms of the formation of a polar activated complex and suggest an I d mechanism. The inhibitory effect of 2,3-dimercaptopropanol on the reaction was also studied and explained in terms of binding of this species to the catalyst. Introduction Among transition metal cyano-complexes, pentacyanofer- rate(II) has been intensively researched in aqueous as well as in micellar media [1–5]. Over the last few decades, hexacyanoferrate(II) complexes have attracted the attention of many researchers [6–13]. These compounds have been studied both with respect to their oxidation kinetics in acidic and neutral medium [11, 12] and also in substitution reactions [6, 7, 13]. Exchange between labelled and free cyanide is very slow in the dark, but under the action of light, reversible aquation takes place with the formation of [Fe(CN) 5 H 2 O] 3- and CN - [14]. Most of the mono-substituted cyano-complexes of iro- n(II) have been obtained either through photochemical aquation [14] or by metal-catalysed dissociation of hexa- cyanoferrate(II) [15] followed by reaction with an incom- ing ligand. Thermal aquation of hexacyanoferrate(II) is a slow reversible process as shown in Eq. 1. Earlier experi- ments have shown that aquapentacyanoferrate(II) reacts with nitrogen heterocycles to give intensively coloured products [16]. The known reaction of hexacyanoferrate(II) with nitrosobenzene [17] suggests a similar reaction with 2-Mepz according to Eqs. (1)–(3). ½Fe(CN) 6  4 þ H 2 O  k 1 k 2 ½Fe(CN) 5 H 2 O 3 þ CN  ðslowÞ ð1Þ ½FeðCNÞ 5 H 2 O 3 þ 2-Mepz !½Fe(CNÞ 5 ð2-MepzÞ 3 þ H 2 O ð2Þ CN  þ H 2 O  HCN þ OH  ð3Þ Mercury is known to catalyse the decomposition of hexacyanoferrate(II) [17] in the following manner: Hg 2þ þ½Fe(CNÞ 6  4 ! H 2 O ½Fe(CN) 5 H 2 O 3 þ HgCN þ ð4Þ HgCN þ þ H þ ! Hg 2þ þ HCN ð5Þ Some transition metals such as Hg(II) and Ag(I) are good homogenous catalysts for the exchange of cyanide by nitrogen-containing heterocycles in [Fe(CN) 6 ] 4- [7, 15]. In acidic medium, Hg 2? is regenerated by the action of protons on HgCN ? . In basic solution, the catalyst is not N. Bansal (&) School of Biological and Chemical Sciences, The University of the South Pacific, Laucala Campus, Suva, Fiji Islands e-mail: nrbansal@yahoo.co.in 123 Transition Met Chem (2009) 34:695–702 DOI 10.1007/s11243-009-9250-8