ELECTRON-TRANSFER REACTIVITY OF CUBOIDAL HETEROMETALLIC MO(3)MS(4) CLUSTERS IN AQUEOUS ACIDIC SOLUTIONS

Rate constants (25 degrees C) are reported for the first time for the oxidation of the cuboidal heterometallic Mo3CoS4 cluster, which in 2 M ClO4-/HClO4 solutions exists predominantly as the edge-linked double cube [{Mo3CoS4(H2O)(9)}(2)](8+). Reactions with one-equivalent oxidants, the outer-sphere reagent [Co(dipic)(2)](-) (E degrees' = 0.75 V) (dipic = pyridine-2,6-dicarboxalate) and hexaaqua Fe(III) (E degrees' = 0.77 V), have been studied, I = 2.00 M (LiClO4). Comparisons are made with rate constants already reported for the same two oxidants with [Mo4S4(H2O)(12)](4+), and heterometallic cuboidal clusters [Mo3CrS4(H2O)(12)](4+), [Mo3FeS4(H2O)(10)](4+), [Mo3NiS4(H2O)(10)](4+), edge-linked [{Mo3PdS4(H2O)(9)}(2)](8+), [Mo3CuS4(H2O)(10)](4+) and [Mo3CuS4(H2O)(10)](5+), all of which are derivatives of the trinuclear incomplete cuboidal Mo(IV)(3) cluster [Mo3S4(H2O)(9)](4+). Rate constants for the reactions of [Co(dipic)(2)](-) (k(Co)) are independent of [H+] in the range 0.50-2.00 M. In the case of the Fe(III) oxidations (k(Fe)), the dependence k(Fe) = k(a) + k(b)[H+](-1) is observed for all the clusters except [Mo4S4(H2O)(12)](4+). The term k(a) is assigned as an outer-sphere reaction of [Fe(H2O)(6)](3+), and k(b) as an inner-sphere reaction of [Fe(H2O)(5)OH](2+). Linear correlations of log k(Co) versus log k(a), and log k(Co) versus log k(b), have slopes close to 1.0. Exceptions are the edge-linked [{Mo3PdS4(H2O)(9)}(2)](8+), and to a lesser extent the k(b) term for [{Mo3CoS4(H2O)(9)}(2)](8+). The correlation of log k(Co) versus log k(b) values is unexpected since k(b) is for an inner-sphere process. It can be concluded that electron transfer following OH- bridging of the Fe(III) to the heterometal atom is rate determining. Rate constants k(Co) and k(Fe) for [W3CuS4(H2O)(10)](5+), the first heterometallic cluster obtained from [W3S5(H2O)(9)](4+), also conform to the same two correlations.