DALTON J. Chem. Soc., Dalton Trans., 1997, Pages 2163–2166 2163 Formation of an adduct between thiocyanate ion and nitrosyl thiocyanate Anne M. M. Doherty,a Michael S. Garley,b Neil Haine a,b and GeoVrey Stedman *,b a Science Department, Worcester College of Higher Education, Henwick Grove, Worcester WR2 6AJ, UK b Chemistry Department, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK Nitrosyl thiocyanate reacted with sufficiently high concentrations of thiocyanate ion to form an adduct, [ON(SCN)2]2, which absorbs strongly in the UV region.This species has different properties to that species described in the literature as [NO(SCN)2]2 formed in pulse-radiolysis experiments involving (SCN)2 2 and NO. The difference probably arises from different structures, an S-nitroso as compared to an N-nitroso compound. Acidic solutions containing both nitrous acid and thiocyanate ions have a marked red colour, due to an absorption maximum at 460 nm.From the variation of A460 with [H1], [HNO2] and [SCN2] it has been established 1 that the red species is nitrosyl thiocyanate, ONSCN, formed as shown in equation (1). This H1 1 HNO2 1 SCN2 ONSCN 1 H2O (1) species has also been isolated at low temperatures 2 by the reaction of ONCl with AgSCN. From these absorbance measurements the formation constant of ONSCN has been calculated, and emax established as 100 dm3 mol21 cm21. These measurements used relatively low concentrations of thiocyanate ion, less than 0.1 mol dm23.Later measurements,3 using much higher values of [SCN2], several mol dm23, showed that although the colour appeared much the same to visual observation the shape of the spectrum changed. The absorbance minimum on the low-wavelength side of 460 nm disappeared and the spectrum was a smooth curve in which the absorbance rose steeply towards low wavelengths in the UV region. Changes on the high-wavelength side of 460 nm were much less marked.It was suggested that this might be due to the formation of a species [ON(SCN)2]2. Recently Czapski et al.4 used pulse radiolysis to study the reaction of NO with a number of radical anions including (SCN)2 2, and observed the formation of a species absorbing strongly in the UV region with e260 ª 7570 dm3 mol21 cm21 which they suggested was [NO(SCN)2]2. This was observed to decay by a process showing first-order kinetics with a rate constant 2 × 104 s21 that was independent of pH, [NO] and [SCN2] to yield a species with a much weaker UV absorption and which was suggested to be NOSCN, as shown in (2).[NO(SCN)2]2 NOSCN 1 SCN2 (2) There were a few measurements of the decay of NOSCN to form NO2 2 and SCN2, with kinetics that had a fractional order in [H1]. A comparison of the results of Czapski et al.4 with our observations indicated a number of points of disagreement and we have therefore extended our own work. Results and Discussion Nitrosyl thiocyanate is unstable in aqueous solution; the red colour fades over a period of 10–20 min with the evolution of bubbles of NO, and in the original work,1 which involved manual spectroscopy, this was allowed for by measuring the absorbance at a number of times and extrapolation to the time of mixing the reactants.This would not, however, detect a rapid initial reaction in the interval between mixing the solutions and measuring the first point. We have therefore used stopped-flow methods, mixing solutions of NaSCN 1 NaNO2 with HClO4, recording the initial absorbance after mixing.Fig. 1 shows a spectrum of ONSCN plotted for a series of repeat runs carried out at selected wavelengths, and the ONSCN peak at 460 nm can be clearly seen. Using the value of e460 = 100 dm3 mol21 cm21, the equilibrium constant K1 = [ONSCN]/[H1][HNO2]- [SCN2] is calculated as 44 dm6 mol22 for 2 mol dm23 ionic strength (NaClO4); this may be compared with earlier values of 21.0, 28.6 and 36.3 for ionic strengths of 0.42, 1.02 and 1.42 mol dm23 obtained by manual spectrometry1 with a Unicam SP500 spectrometer.The figure also shows a trace obtained at a much higher thiocyanate ion concentration, [SCN2] = 0.5 mol dm23, where it can be seen that the absorption on the low-wavelength side of 460 nm has increased markedly, with much smaller increases on the high-wavelength side. The value of A460 is significantly higher than that expected for K1 = 44 dm6 mol22 and [SCN2] = 0.5 mol dm23, confirming the presence of another species.Unfortunately the limitations of our stopped-flow apparatus made it impossible to extend these measurements into the UV region. Measurements by the stopped-flow method made at a single wavelength, 460 nm, and with varying [SCN2] are shown in Fig. 2. On the basis of K1 = 44 dm6 mol22 there ought to be 81% conversion of nitrite into ONSCN at [SCN2] = 1 mol dm23 and the absorbance vs. [SCN2] curve would be expected to be levelling off; instead the absorbance increases steadily, indicating the presence of another species.To extend our measurements to lower wavelengths we turned to conventional spectrophotometry. A spectrum of ONSCN Fig. 1 Visible spectra of ONSCN obtained by stopped-flow spectrophotometry at pH 1 and 25 8C: (m) [SCN2] = 0.02, [HNO2] = 0.02 mol dm23; (d) [SCN2] = 0.5, [HNO2] = 0.02 mol dm232164 J. Chem. Soc., Dalton Trans., 1997, Pages 2163–2166 obtained at low [SCN2] is shown in Fig. 3. The rate of decomposition of nitrosyl thiocyanate is a very sensitive function of concentration. To stabilise the solutions we reduced the nitrite concentration to 1024 mol dm23, and used 4 cm cells to obtain reasonable absorbances. Typical spectra are shown in Fig. 4, where it can be seen that as [SCN2] increases the UV absorption increases rapidly, filling the minimum below 460 nm, so that the peak characteristic of ONSCN disappears. Plots of A vs.[SCN2] show a linear increase in absorbance, typical values being included in Fig. 2. In making these measurements it was important to record the spectrum as soon as possible after mixing to avoid complications due to formation of the heterocycle iso-perthiocyanic acid (also known as xanthane hydride), 5-amino-1,2,4-dithiazole-3- thione (C2H2N2S3). The rate of formation of this yellow species is a very sensitive function of [SCN2], and Wilson and Hall 5 have shown that the rate of its formation is n = k[H1][SCN2]3.It has a very characteristic VIS/UV spectrum with fine structure peaks, and is readily identifiable. No trace of its spectrum was seen in our work, or in the published spectra of Czapski et al.4 However at our highest concentration of thiocyanate this compound precipitated on standing, and was identified by mass5 and infrared spectroscopy.6 We interpret our VIS/UV spectra as showing the formation of an additional species by the reaction of SCN2 with ONSCN.The increase in absorbance with [SCN2] is almost linear, suggesting that one thiocyanate ion is involved; as there is no sign of levelling off it seems likely that under our conditions only a small fraction of the ONSCN has been converted into the new species. We suggest the reaction is shown in equation (3), ONSCN 1 SCN2 [ON(SCN)2]2 (3) Fig. 2 Variation of absorbance with [SCN2] for solutions of H1 1 HNO2 1 SCN2 at 25 8C by stopped-flow (m) and UV spectrophotometry (d) (m): [H1] = 0.1, [HNO2] = 0.02 mol dm23; l = 2 mm; l = 460 nm; (d) [H1] = 0.1, [HNO2] = 0.0001 mol dm23; l = 4 cm; l = 320 nm Fig. 3 The UV/VIS spectrum of ONSCN at [H1] = 0.1, [HNO2] = 0.01, [SCN2] = 0.01, I = 2 mol dm23 and l = 4 cm without specifying a structure for [ON(SCN)2]2. If we assign formation constants K1 and K2 and absorption coefficients e1 and e2 to the species ONSCN and [ON(SCN)2]2 then the apparent absorption coefficient eobs is shown in equation (4), eobs = A/[nitrite]l = e1K1[H1][SCN2] 1 e2K1K2[H1][SCN2]2 1 1 K1[H1][SCN2] 1 K1K2[H1][SCN2]2 (4) where [nitrite] = [HNO2] 1 [ONSCN] 1 [ON(SCN)2 2] and l is the path length.Fitting data of the type shown in Fig. 2 by equation (4) has limitations because the measurements do not extend to concentrations where A begins to level off for substantial conversion into [ON(SCN)2]2, and this introduces large error limits on the constants deduced. In addition, although there is formal constant ionic strength, at concentrations of 2 mol dm23 the activity effects of NaClO4 and NaNCS will be different; thus the mean ion activity coefficients at a molality of 2 are 0.744 (NaNCS) and 0.609 (NaClO4).We assigned e1 = 100 dm3 mol21 cm21 as this is well established. Fitting the stopped-flow data at 460 nm by equation (4) we obtain 1023e2 = 1.74 ± 0.54 dm3 mol21 cm21, K1 = 33.4 ± 4.6 dm6 mol22 and 102K2 = 3.6 ± 1.3 dm3 mol21. The value of K1 is in fair agreement with earlier measurements and serves as a check on the curve fitting, but there are substantial errors associated with both K2 and e2.Such calculations are very sensitive to the values assigned to parameters in the equation. If, in addition to assigning e = 100 dm3 mol21 cm21 we also fix K1 as 44 dm6 mol22 then 1023e2 = 1.77 ± 0.71 dm3 mol21 cm21 and 102K2 = 1.32 ± 0.59 dm3 mol21. As the increase in absorbance with [SCN2] is almost linear we have only a small conversion into [ON(SCN)2]2 and we are virtually measuring K2e2 and there is not sufficient curvature to assign precise separate values to K2 and e2, though it is clear that K2 is small and e2 large (compared to K1 and e1 respectively).We now turn to a comparison of the present results with those of Czapski et al.4 Their evidence for the formation of a species with an intense UV spectrum by an encountercontrolled reaction between NO and (SCN)2 2 is very strong and [NO(SCN)2]2 seems a likely formulation.It must however be a different species to that formed by the reaction of SCN2 with ONSCN. The species [NO(SCN)2]2 decomposes with a firstorder rate constant of 2.1 × 104 s21 (t2� 1 = 3.3 × 1025 s), and would have completely decomposed long before our first measurements, even by stopped flow. Nevertheless the UV evidence for the presence of a species in our work is unmistakable and we suppose that they must be different compounds, possibly isomers. For this reason we write the formulae of the Czapski species in the same way as in their original paper, [NO(SCN)2]2 Fig. 4 The UV/VIS spectra at various [SCN2], [H1] = 0.1, [HNO2] = 0.0001, I = 4 mol dm23 and l = 4 cm. [SCN2] = 2.5 (A), 2.0 (B), 1.5 (C), 1.0 (D), 0.5 (E), 0.25 (F) and 0.1 (G) mol dm23. The spectra are shown in sequence A æÆ GJ. Chem. Soc., Dalton Trans., 1997, Pages 2163–2166 2165 Table 1 Variation of e420 with [thiourea] at 25 8C [(NH2)2CS]/mol dm23 e420 a/dm3 mol21 cm21 0.035 113 0.072 113 0.147 118 0.222 119 0.297 120 0.409 126 0.522 132 0.709 132 0.897 139 [(NMe2)2CS]/mol dm23 e420 b/dm3 mol21 cm21 0.02 192 0.06 196 0.10 199 0.14 204 0.18 208 a [H1] = 0.10, I = 0.3, [(NH2)2CSNO1] = 0.003 16 mol dm23.b [H1] = 0.49, I = 0.5, [(NMe2)2CSNO1] = 0.006 37 mol dm23. and NOSCN, whereas we write our species as [ON(SCN)2]2 and ONSCN emphasising that the nitroso group is bonded to sulfur. Czapski et al.4 suggested that [NO(SCN)2]2 decomposed to NOSCN 1 SCN2, and they show a spectrum for NOSCN that rises smoothly towards lower wavelengths in the UV region.Unfortunately their measurements do not extend above 340 nm, so it is not known whether the characteristic peak of ONSCN at 460 nm appears. The general shape of the UV spectrum is similar to our own observation on ONSCN, but the absorption coefficient at 320 nm is ª190 dm3 mol21 cm21, much lower than our own value of 924 dm3 mol21 cm21. Again it appears that they may be observing a different species.The hydrolysis reaction of the Czapski species NOSCN was found to obey pseudo-first-order kinetics, with a rate constant that decreased with increase of pH, 730 (pH 5.9), 458 (6.8), 126 (7.9) and 1.3 s21 (1.5). This was interpreted by the mechanism in equations (5) and (6) with k6 ª 800 s21 and k25/k6 ª 107. There NOSCN 1 H2O (HO]NOSCN)2 1 H1 (5) (HO]NOSCN)2 1 H1 HNO2 1 SCN2 (6) are some unusual features here. Reaction (6) is written as a bimolecular process, but k6 is quoted as a first-order rate constant.The value of 800 s21 looks to be a limiting value of the observed pseudo-first-order rate constant at high acidity. It can be compared with the value obtained by conventional methods for the hydrolysis of ONSCN. The rate constant for the forward reaction (1) is 7 11 700 dm6 mol22 s21, and this value is supported by an independently determined value 8 at 0 8C which, when corrected for temperature variation (E = 54 kJ mol21), gives 10 900 dm6 mol22 s21 at 25 8C.The equilibrium constant 1 for (1) is 32 dm6 mol22, giving the rate constant for the hydrolysis reaction of 365 s21. The difference between this figure and 800 s21 may be significant, but is not sufficiently large to be certain. The reasons for the differences between our results and those of Czapski et al.4 seem most likely to lie in different structures. Czapski did not, of course, on the basis of only UV evidence suggest a structure for [NO(SCN)2]2, but as we propose that we observe an isomer it is incumbent on us at least to make a suggestion.The structure of thiocyanogen (SCN)2 certainly involves a sulfur–sulfur bond, and it seems likely that (SCN)2 2 will be [NCS]SCN]2. An encounter-controlled reaction with NO? could lead to the formation of an N-nitroso species [NCS]SCN?NO]2. The sulfur–nitroso compounds (thionitrites) are characterised by red/yellow colours and absorption in the visible region, and nitrosyl thiocyanate clearly belongs in this class with an S]N]] O chromophore. We do not know the structure of our adduct, but if it is derived from ONSCN then it is likely that it will have an S]NO bond and hence be different from [NCS]SCN?NO]2 and have a different spectrum and different stability.The decomposition of [NCS]SCN?NO]2 with loss of SCN2 will presumably produce nitrosyl isothiocyanate SCNNO, which will differ from our species ONSCN. Detailed calculations have been published by Westwood and Pasinszki 9 of the stability of open-chain and ring isomers of composition CN2OS.They calculate that the gasphase species SCNNO is some 16–17 kJ mol21 less stable than ONSCN. Both are anti structures. These calculations indicate that the differences in stability of the N- and S-nitroso isomers is small enough that there is nothing unreasonable in postulating this as the structure of Czapski’s compound, thus accounting for the differences between his results and ours.In conclusion we cite some other evidence for the existence of other disulfide species with sulfur bonded to a nitroso group. Nitrous acid reacts with thiourea to form10 a yellow S-nitroso product, (NH2)2CSNO1, as shown in equation (7), with a (NH2)2CS 1 H1 1 HNO2 (NH2)2CSNO1 1 H2O (7) formation constant of ca. 5000 dm6 mol22. The stability of this species has been extensively examined11 by stopped-flow methods at 420 nm, and its chemistry as a nitrosating agent has been studied in detail by Dix and Williams,12 it shows a very similar reactivity pattern to that of ONSCN. The absorption coefficient at 420 nm of solutions of thiourea 1 nitrous acid in which there is essentially 100% conversion of nitrite into (NH2)2CSNO1 increases markedly and linearly with increase in [(NH2)2CS].Similar behaviour is observed for solutions of (NMe2)2CS 1 nitrous acid. Data are shown in Table 1. These observations parallel the effects observed in the HNO2–SCN2 system, but in this case there is kinetic evidence that is consistent with the presence of a species [(NH2)2CS]2- NO1, equation (8).The rate of decay of (NH2)2CSNO1 (NH2)2CSNO1 1 (NH2)2CS K3 [(NH2)2CS]2NO1 (8) has been measured,11 and in the presence of a large excess of thiourea the rate law (9) is observed. This was originally 2d[(NH2)2CSNO1]/dt = k2[(NH2)2CSNO1][(NH2)2CS] (9) interpreted as attack by thiourea on (NH2)2CSNO1 to form (NH2)2CSSC(NH2)2~1 1 NO?, the radical cation undergoing further oxidation to form the known product (NH2)2CSSC- (NH2)2 21.This does not account however for the increase in absorbance with [(NH2)2CS]. However if there is the formation of a small amount of a thiourea adduct which absorbs much more strongly than (NH2)2CSNO1 and which decomposes then precisely the same rate law would be observed, the difference ing that k2 would be K3k3, equations (8) and (10). [(NH2)2CS]2NO1 k3 (NH2)2CSSC(NH2)2~1 1 NO (10) The structure of the postulated adduct is not known, but as the end product of the decomposition of (NH2)2CSNO1 is (NH2)2CSSC(NH2)2 21 which is known from X-ray crystallography 13 to contain an S]S linkage it is reasonable to assume that this exists in the adduct also.We considered the possibility that the rate law (9) might be due to nitrosation of a dimer of thiourea, but investigation of the 13C NMR shift of aqueous solutions of thiourea showed no sign of any variation with [(NH2)2CS].Similarly we were unable to detect any deviation from the Beer–Lambert law for thiourea solutions, so we did not find any evidence for dimer formation.14 We conclude that sulfur–nitroso compounds can undergo addition reactions with other sulfur nucleophiles to form adducts in which a sulfur–sulfur bond is formed. It seems likely2166 J. Chem. Soc., Dalton Trans., 1997, Pages 2163–2166 that the species [NO(SCN)2]2 and NOSCN observed in pulse radiolysis are N-nitroso compounds, and that the compound described by Czapski et al.4 as NOSCN is different from the nitrosyl thiocyanate known as a nitrosating agent in aqueous solutions.Experimental Materials All chemicals were of AnalaR grade with the exception of tetramethylthiourea and were used without further purification. Spectrophotometric and kinetic measurements Stopped-flow measurements were made with a Hi Tech Canterbury SF-3A instrument, fitted with a data-collection system as described previously.11 Conventional VIS/UV spectra were measured on a Perkin-Elmer lambda nine spectrometer about 30 s after mixing.The spectrum of ONSCN, Fig. 1, was obtained from a solution of H1 1 HNO2 1 SCN2 at low [SCN2] in order to avoid complications due to formation of [ON(SCN)2]2. Under these conditions there is only partial conversion into nitrosyl thiocyanate and corrections for absorption due to HNO2 and SCN2 were subtracted. Absorption coefficients were calculated by taking the absorbance at 460 nm to be equivalent to e = 100 dm3 mol21 cm21. Acknowledgements We acknowledge financial support from Worcester College of Higher Education (to N. H.). References 1 G. Stedman and P. A. E. Whincup, J. Chem. Soc., 1965, 4813. 2 C. C. Addison and J. Lewis, Q. Rev. Chem. Soc., 1955, 9, 115. 3 C. G. Munkley, Ph.D. Thesis, University of Wales, 1990. 4 G. Czapski, J. Holcman and B. H. J. Bielski, J. Am. Chem. Soc., 1994, 116, 11 465. 5 I. R. Wilson and W. H. Hall, Aust. J. Chem., 1969, 22, 513. 6 H. J. Emeleus, A. Haas and N. Shepherd, J. Chem. Soc., 1963, 3165. 7 J. Fitzpatrick, T. A. Meyer, M. E. O’Neill and D. L. H. Williams, J. Chem. Soc., Perkin Trans. 2, 1984, 927. 8 G. Stedman, J. Chem. Soc., 1959, 2949. 9 N. P. C. Westwood and T. Pasinszki, J. Chem. Soc., Faraday Trans., 1996, 333. 10 K. Y. Al-Mallah, P. Collings and G. Stedman, J. Chem. Soc., Perkin Trans. 2, 1975, 1734. 11 P. Collings, M. S. Garley and G. Stedman, J. Chem. Soc., Dalton Trans., 1981, 331; M. S. Garley, G. Stedman and H. Miller, J. Chem. Soc., Dalton Trans., 1984, 1959. 12 L. R. Dix and D. L. H. Williams, J. Chem. Res., 1982, (S) 190. 13 O. Foss, J. Johnsen and O. Tvedten, Acta Chem. Scand., 1958, 12, 1782. 14 M. S. Garley, Ph.D. Thesis, University of Wales, 1982. Received 14th February 1997; Paper 7/01059A