Pw. Dimmock et al., ELECTRON-TRANSFER REACTIVITY OF CUBOIDAL HETEROMETALLIC MO(3)MS(4) CLUSTERS IN AQUEOUS ACIDIC SOLUTIONS, Inorganica Chimica Acta, 225(1-2), 1994, pp. 157-162
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.