EFFECT OF SUBSTITUTING SULFUR FOR OXYGEN ON THE HETEROGENEOUS ELECTRON-TRANSFER KINETICS OF OXOMOLYBDENUM(V) HYDROTRIS(3,5-DIMETHYL-1-PYRAZOLYL)BORATE COMPLEXES

Heterogeneous electron transfer rate constants (k(s,h)) and formal pot entials (E degrees') are measured for the one-electron reduction of a series of oxomolybdenum(V) complexes, HB(Me(2)pz)(3)MoO(L-L), where HB (Me(2)pz)(3)(-) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and L-L(2- ) is a bidentate 1,2-disubstituted ethane or benzene ligand whose dono r atoms can be either O or S. E degrees' shifts in the negative direct ion by about 1 V and k(s,h) decreases by about 1.5 orders of magnitude as S is replaced by O in the inner coordination sphere of these compo unds. These observations are rationalized in terms of a hard acid-hard base interaction between the HB(Me(2)pz)(3)MoO2+ fragment and L-L(2-) which stabilizes the metal-ligand bond and increases the energy of th e metal-centered redox orbital as the hard base character of the biden tate ligand increases. Electron transfer rate differences are believed to arise primarily from larger inner-shell contributions to the activ ation energy barrier for Mo-O versus Mo-S complexes, but contributions from differences in outer-shell energies and adiabaticities cannot be eliminated at this time.