ELECTRON-TRANSFER-120 - REDUCTIONS OF THE CUBOIDAL MIXED-VALENCE MOLYBDENUM SULFUR CLUSTER, [MO4S4(H2O)(12)](5+)/

The cuboidal mixed-valence Mo/S cluster, [Mo4S4(H2O)(12)](5+) (I), in which each of the four equivalent h-lo atoms assumes a mean oxidation state of 3.25, is reduced to the isostructural Mo(III) tetramer, [Mo4S 4(H2O)(12)](4+), by equimolar quantities of Ti(III), Cr(II), Eu(II), V (II), Ru(NH3)(6)(2+) and Co-II(sepulchrate), as well as by radicals de rived from flavin-like heterocycles and by dihydro derivatives of thes e heterocycles. Reaction rates are independent of H+ (within the range 0.5-2.0 M H+) for the metal center reductants and for the radicals, b ut feature prominent [H+](-1)-proportional terms for the dihydro compo unds. All reactions appear to be outer-sphere. The deprotonated form o f the dihydroriboflavin reacts 600 times more rapidly than the corresp onding radical, a selectivity similar to that previously observed with an array of Co(lII) oxidants, Specific rates for reductions by the me tal-ion centers, in conjunction with the treatment of Marcus, lead to a self-exchange rate 10(3.8+/-1.6) M(-1) s(-1) for the couple [Mo4S4]( 5+/4+). The latter lies very close to that recorded for the truncated cuboidal trinuclear OH-bridged system, Mo-IV,Mo-III,Mo-III/(Mo-III)(3) . It is possible that the observed correspondence in rates reflects a similarity in net valence change in the absence of major alterations i n bond connectivities or in molecular geometry.