Equilibrium Isotope Effects for H2 versus D2 Binding

Equilibrium constants determined at 13-36°C by IR measurements of Vco show that the thermochemical parameters for binding of H2 are AH° = — 6.8 0.5 kcal/mol and AS° = —24.7 2.0 e.u. (these newer values are more accurate than those in Table 7.5). For binding of D2, AH° = -8.6 0.5 kcal/mol and AS° = -30.0 2.0 e.u., i.e., D2 binds better enthalpically (AAH = 1.8 kcal/mol), which easily overcomes the disfavored entropy of D2 complexation (AA5 = 5.3 cal/mol deg). This domination of enthalpy over entropy is general to these primary isotope effects (unlike, e.g.,H2 versus N2 binding), and thus EIEs are enthalpically driven. It should be kept in mind that EIEs are temperature-dependent because of the entropy differences. 

EIE for the W analogue cannot be measured directly because of stronger WCHj) bonding, but the equilibrium shown in Eq. (7.32) provides a means of determining accurate EEE values  

Spectroscopic measurements using cahbrated Hj/Nj and Dj/Nj gas mixtures give KJKjy = 0.70 0.15 in THE solvent at 22°C. 

Data for Hj versus Dj binding are reported for Ir and Os complexes that have both hydride and Hj ligands, i.e., MH,j(H2)L,. Lower of 0.36-0.50 occur 

1 kcal/mol higher thanH2 loss from IrClH2(H2)(L)2 d = P Bu2Me).  

---