THERMODYNAMICS OF HETEROLYTIC AND HOMOLYTIC M-H 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)3H and Tp'M(CO)3H (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)3H 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)3H > TpM(CO)3H > Tp'M(CO)3H. This trend is the opposite of that expected on the basis of the relative electron-richness of the metal centers, as measured by infrared nu(CO) frequencies and oxidation potential data for the corresponding metal anions. The homolytic bond dissociation energies (BDEs) of the complexes in acetonitrile solution were obtained by the use of a known thermochemical cycle based on the pK(a) and anion oxidation potential data. The BDE values decrease in the order CpM(CO)3H > TpM-(CO)3H > Tp'M(CO)3H. Steric effects significantly contribute to the ordering of the heterolytic and homolytic M-H bond strengths. The tendency of the Tp ligand to electronically favor octahedral coordination may also be of importance. Thermochemical cycles were used to probe the effect of a one-electron oxidation of the metal hydrides on the strengths of the Mo-H and W-H bonds. The oxidations led to a weakening of the bonds by a relatively constant 108-113 kJ/mol toward deprotonation, and by 25-33 kJ/mol toward homolysis.