Thermodynamic and Kinetic Acidity of H2 Ligands

The pKa values of H2 complexes are rarely measured in aqueous solution but are usually discussed on an adjusted pseudoaqueous scale for comparative purposes because P Ka values determined by solution equilibrium measurements are solvent-dependent. As discussed in reviews by Morrisr and Jia2 and subsequent work,43 48 49 P K. values are usually determined by NMR measurement of the concentrations of species in an equilibrium  

Thus the predominant tautomer is always the weaker add, and interestingly, the apparent pJCe of rapidly interconverting tautomers is larger than the pKB of either tautomer alone.48 

7 Hz in these examples, and other properties of H2 ligands. Most of the highly acidic complexes are found for Re and especially Os, although less stable Ru analogues exist in some cases.56,57,68 

The electrochemical potential E1/2 for ds/d6 couples is a good diagnostic for electron density at a d6 M, where a low Em corresponds to an electron-rich M. Based on the work ofTilset and Parker59 for hydrides, Morris1,36,52 has related the pKa of a cationic H2 complex [M(H2)L5]+ to the electrochemical potential of the conjugate base MHL5 by the thermochemical cycle  

These terms combine to give the free energy of H-atom abstraction in Eq. (9.33) (BDE = bond dissociation energy), which can be used to calculate pseudoaqueous p Ka values by rearrangement, conversion to enthalpies instead of free energies,59 and use of an empirical constant  

---