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 oxidant s, the outer-sphere reagent [Co(dipic)(2)](-) (E degrees' = 0.75 V) (d ipic = 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 [M o4S4(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 cu boidal Mo(IV)(3) cluster [Mo3S4(H2O)(9)](4+). Rate constants for the r eactions of [Co(dipic)(2)](-) (k(Co)) are independent of [H+] in the r ange 0.50-2.00 M. In the case of the Fe(III) oxidations (k(Fe)), the d ependence 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-sp here reaction of [Fe(H2O)(6)](3+), and k(b) as an inner-sphere reactio n 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. Exceptio ns are the edge-linked [{Mo3PdS4(H2O)(9)}(2)](8+), and to a lesser ext ent the k(b) term for [{Mo3CoS4(H2O)(9)}(2)](8+). The correlation of l og k(Co) versus log k(b) values is unexpected since k(b) is for an inn er-sphere process. It can be concluded that electron transfer followin g OH- bridging of the Fe(III) to the heterometal atom is rate determin ing. Rate constants k(Co) and k(Fe) for [W3CuS4(H2O)(10)](5+), the fir st heterometallic cluster obtained from [W3S5(H2O)(9)](4+), also confo rm to the same two correlations.