Thermal substitution reactions, volume

A = +14 cm3 mol-1 for both the forward and the reverse reaction. That this AV value is markedly less than the partial molar volumes of water and of ammonia (25 and 18 cm3 mol-1, respectively) indicates limiting dissociative (D) activation (133), as do the A values of close to +70JK-1mol-1 in both directions. Overall, the current situation with regard to thermal substitution at pentacyanoferrates(II) appears to be that an I,i mechanism can also operate for reactions of [Fe(CN)5(H20)]3-, whereas the D mechanism operates for all other [Fe(CN)5L]" complexes (134). 

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

Both of these complexes are known to undergo thermal substitution near 100 C via clean first-order kinetics Q4. IS- Thus we performed the equilibrium experiments between 80 and 120 C. Since the equilibrium constant was expected to heavily favor the SBu2 complex 2 we ran the experiments with high concentrations of cw-cyclooctene, in either neat cw-cyclooctene (7.7 M) or 1.2 M solutions of cw-cyclooctene in heptane. The results obtained in both solvent systems were identical. Although differential vaporization of the ligands did not appear to be a problem, efforts were made to minimize the gas volume in the reaction vessel. 

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