Thermodynamics of heterolytic and homolytic metal-hydrogen bond cleavage reactions of 18-electron and 17-electron group 6 hydridotris(pyrazolyl)borate metal hydrides

The energetics of the deprotonation and metal-hydrogen bond homolysis reactions of TpM(CO) 3 H and Tp'M(CO) 3 H (Tp=hydridotris(pyrazolyl)borate; Tp'=hydridotris(3,5-dimethylpyrazolyl)borate; M=Cr, Mo, W) and their cation radicals have been investigated, and the results are compared with available data for the analogous CpM(CO) 3 H complexes. Proton-transfer equilibrium measurements have established that for a given metal, acetonitrile pK a values of the metal hydrides decrease in the order CpM(CO) 3 H>TpM(CO) 3 H>Tp'M(CO) 3 H. This trend is the opposite of that expected on the basis of the relative electron-richness of the metal centers, as measured by infrared ν CO frequencies and oxidation potential data for the corresponding metal anions