| Abstract
| - Conversion of the edge-linked (Pd−S on onecube to S−Pd on the other) double cube[{Mo3PdS4(H2O)9}2]8+to the single cube[Mo3(PdX)S4(H2O)9]4+is induced by X = CO, two water-soluble phosphines (which complex atthe Pd), and Cl-, Br-, andNCS-, which also substitute at the Mo's givingadditional kinetic steps. All the reagents show high affinity forand react rapidly at the tetrahedral Pd to give single-cube structures,whereas H2O coordinated to the Pd is not able to retain thesingle-cube structure. Conversion of the single cube[Mo3(PdX)S4(H2O)9]4+to the double cube is brought about by chromatographic or chemicalremoval of X.
- The kinetics of conversion of an edge-linked double cube, in thiscase[{Mo3PdS4(H2O)9}2]8+,to the correspondingsingle cube[Mo3(PdX)S4(H2O)9]4+,has been studied for the first time. Reaction is induced by sixreagents X =CO, two water-soluble phosphines, Cl-, Br-,and NCS-, which complex at the tetrahedral Pd. Thefirst stage ofreaction is fast and is accompanied by color changes, e.g. purple todark blue in the case of Cl-, assigned asdouble to single cube conversion. With X = CO and the twophosphines, when absorbance changes are intenseenough for stopped-flow monitoring with reactants at ≤1 mM, rateconstants 10-5k/M-1s-1 at 25 °C are asfollows: CO, 1.11; PTA, 27.8;P(C6H4SO3)33-,9.6; at I = 2.00 (Li(pts)). The reactions areindependent of [H+]in the range 0.30−2.00 M, and no substitution at the Mo's isobserved. The first stages with X = Cl-,Br-, andNCS- were too fast to monitor, but equilibrium constantsK1/M-1 weredetermined, Cl- (490), Br- (8040),andNCS- (630), by UV−vis spectrophotometry. Twosubsequent kinetic stages are assigned to substitution attheMo's. Similar behavior is observed for[Mo3FeS4(H2O)10]4+,which was selected because substitution at the Feis also fast and there is no known double-cube formation. For bothMo3Pd and Mo3Fe the latter two stagescanbe explained by substitution at nonidentical (two α and one β)H2O's on each Mo or by the presence of mixed-valent MoIII2MoIV forms whichare sufficiently long-lived to give a kinetic discrimination. Inthe case of NCS-an additional step, 0.015 s-1, independent of[NCS-] is assigned to the isomerization Pd−NCS →Pd−SCN. Onremoval of e.g. Cl- by chromatography or addition ofAg+, the double cube re-forms.
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