Broensted Acidities of Carbonyl Hydride Complexes of Iron, Ruthenium, and Osmium: pKa Values and Deprotonation Rates in Methanol Solution

Article abstract of DOI:10.1021/ja00343a017

Reported are kinetics studies of the reaction of methoxide ion with several mononuclear and polynuclear carbonyl hydride complexes HxMy(CO)z of the iron triad.Deprotonation of the hydrides to give the conjugate base anions occurs with moderate second-order rate constants (ca. 10¹-10⁴ M^-1s^-1), considerably smaller than those of oxygen or nitrogen acids of comparable thermodynamic strength under similar conditions.The relative slowness of these reactions is interpreted in terms of possible electronic and structural rearrangements of substrate and solvent occuring as the result of the deprotonation.In addition the kinetics technique allows the measurement of the reverse reacton (protonation of the conjugate base by methanol) rate constants which are also comparatively small (ca. 0.05-25 s^-1).The ratio of the forward and reverse rates gives the equilibrium constants from which the pK_a's in methanol can be calculated.In several cases pK_a's were confirmed by buffer/indicator techniques.From these and other published data are drawn the conclusions that bridging hydrides are more acidic than terminal hydrides and that hydrides of the lighter metals are more acidic than those of homologous heavier metals.Furthermore, it is shown than the substituent effect of replacing CO of a cluster by a trimethyl phosphite is to decrease significantly the acidity of that cluster's hydrides.