Isomerism of thiocyanate bonded

D. A. Buckingham, Linkage isomerism of thiocyanate bonded to cobalt(IIl), in Coordination Chemistry, A Century of Progress , ACS Symposium Series 565, ed. G. B. Morgan Kaufmann, American Chemical Society, Washington, DC, 1994, p. 256. 

Linkage isomerization of thiocyanate in [Pd(Et4dien)(SCN)]+ takes place more rapidly than that of selenocyanate in [Pd(Et4dien)(SeCN)]+. This observation is consistent with a dissociative mechanism, with relief of steric strain in transition-state formation, for isomerization. The activation enthalpy for the isomerization of [Pd(Et4dien)(SCN)]+ to the N-bonded thiocyanato-isomer is 19.8 kcal mol and for [Pd(Et4dien)(SeCN)]+ to the N-bonded isomer it is 21.0 kcal mol the respective activation entropies are -10 and — 5 cal deg mol L ... 

An interesting use is made of addition to a double bond by glutathione-dependent cis-tmns isomerases.76 One of them converts maleate to fumarate with a turnover number of 300 s"1. Similar enzymes, which participate in bacterial breakdown of aromatic compounds (Fig. 25-7), isomerize maleylacetoacetate and maleylpyruvate to the corresponding fumaryl derivatives (Eq. 13-20). The - SH group of bound glutathione is thought to add to the double bond. Rotation can then occur in the enolic intermediate. Thiocyanate ion catalyzes the isomerization of maleic acid nonenzy-matically, presumably by a similar mechanism. 

Numerous additional examples of linkage isomerization in solids have been reported.13 Most studies have focused on elucidating the relative roles played by steric and electronic effects in determining the bonding mode of the nitrite or thiocyanate ligand. Interestingly, the bonding mode of thiocyanate has been found to be subject to counter-ion control in the solid state.20... 

Isothiocyanates (formerly known as mustard oils, 8-136) are compounds with a cumulated system of double bonds belonging to the heterocumulenes, compounds that are formally derived from the hydrocarbon allene (H2C=C=CH2). In small quantities, isothiocyanates are often accompanied by isomeric thiocyanates. Related cyanates and isocyanates do not occur in foods. 

Tracer studies have shown that the isomerization of /ra/zj-[Co(en)2(NH3)-(SCN)] to the A-thiocyanato-linkage isomer in basic aqueous solution is intramolecular. The detailed mechanism may involve an intimate ion pair which does not exchange its thiocyanate with the thiocyanate in solution, a species in which the thiocyanate is Ji-bonded to the cobalt, or intramolecular attack of a deprotonated amine nitrogen at the thiocyanate carbon (Scheme 5). Aquation of [Co(NH3)5(sulphamate)] - is preceded by isomerization of the starting complex to an equilibrium mixture of N- and 0-bonded isomers. The specific rate constant for approach to equilibrium is l.lxlO s at 25 °C composite activation parameters are AH = 24.7 kcal mol and AS = 10.9 cal deg mol . Inability to estimate the equilibrium constant satisfactorily precludes the separation of these kinetic parameters into their forward and backward components. ... 

Similar isomerizations have been noted for a number of complexes. As with metal nitrosyls, IR spectra can be used to indicate the manner of bonding, but there is an overlap region around 2080-2100 cm-1 where i/(C-N) is found for both N- and S-bonded thiocyanates (additionally, S-bonded thiocyanates usually give a much sharper i (C-N) band). 14N NQR has been shown to be a reliable discriminator, but X-ray diffraction is ultimately the most reliable method. 

The yellow form is stable at room temperature but isomerizes on warming in the solid state or solution. EXAFS measurements indicate that the yellow form has Pt bound to N and S (i.e. the thiocyanate is S-bonded) while the red form has no Pt-S bonds (Figure 3.77) therefore, the thiocyanate is N-bonded (there are also indications of distant Pt-Pt contacts (3.2 A), possibly by stacking of the planar Pt(bipy)(NCS)2 units). 

Linkage isomerism This is a special type of structural isomerism in which the differences arise from a particular ligand which may coordinate to a metal ion in more than one way. In Table 1-3 we indicated that a ligand such as thiocyanate could bond to a metal through either the nitrogen or the sulfur atom, and the complex ions [Co(NH3)5(ACS)]2+ and [Co(NH3)5(5CN)]2+ are related as linkage isomers. 

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