THERMODYNAMICS OF HETEROLYTIC AND HOMOLYTIC M-H BOND-CLEAVAGE REACTIONS OF 18-ELECTRON AND 17-ELECTRON GROUP-6 HYDRIDOTRIS(PYRAZOLYL)BORATEMETAL-HYDRIDES

The energetics of the deprotonation and metal-hydrogen bond homolysis reactions of TpM(CO)3H and Tp'M(CO)3H (Tp = hydridotris(pyrazolyl)bora te; Tp' = hydridotris(3,5-dimethylpyrazolyl)borate; M = Cr, Mo, W) and their cation radicals have been investigated, and the results are com pared with available data for the analogous CpM(CO)3H complexes. Proto n-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 oxidati on potential data for the corresponding metal anions. The homolytic bo nd dissociation energies (BDEs) of the complexes in acetonitrile solut ion 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 signi ficantly contribute to the ordering of the heterolytic and homolytic M -H bond strengths. The tendency of the Tp ligand to electronically fav or octahedral coordination may also be of importance. Thermochemical c ycles 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 oxidat ions 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.