Aqueous Solution Chemistry of the Mo₃PdS₄ Cube: Substitution Reactions and the Double to Single Cube Interconversion Induced by CO, Two Phosphines, Cl^-, Br^-, and NCS^-

The kinetics of conversion of an edge-linked double cube, in this case [{Mo₃PdS₄(H₂O)₉}₂] ⁸⁺, to the corresponding single cube [Mo₃(PdX)S₄(H₂O)₉]⁴⁺, has been studied for the first time. Reaction is induced by six reagents X = CO, two water-soluble phosphines, Cl^-, Br^-, and NCS^-, which complex at the tetrahedral Pd. The first stage of reaction is fast and is accompanied by color changes, e.g. purple to dark blue in the case of Cl^-, assigned as double to single cube conversion. With X = CO and the two phosphines, when absorbance changes are intense enough for stopped-flow monitoring with reactants at ≤ 1 mM. rate constants 10^-5 k/M^-1 s^-1 at 25 °C are as follows: CO, 1.11; PTA, 27.8; P(C₆H₄SO₃)₃^3-.,3-, 9.6; at I = 2.00 (Li(pts)). The reactions are independent of [H⁺] in the range 0.30-2.00 M, and no substitution at the Mo's is observed. The first stages with X = Cl^-, Br^-, and NCS^- were too fast to monitor, but equilibrium constants K₁/M^-1 were determined, Cl^- (490), Br^- (8040), and NCS^- (630), by UV-vis spectrophotometry. Two subsequent kinetic stages are assigned to substitution at the Mo's. Similar behavior is observed for [Mo₃FeS₄(H₂O)₁₀]⁴⁺, which was selected because substitution at the Fe is also fast and there is no known double-cube formation. For both Mo₃Pd and Mo₃Fe the latter two stages can be explained by substitution at nonidentical (two α and one β) H₂O's on each Mo or by the presence of mixed-valent Mo₂^IIIMo^IV forms which are sufficiently long-lived to give a kinetic discrimination. In the case of NCS^- an additional step, 0.015 s^-1, independent of [NCS^-] is assigned to the isomerization Pd-NCS → Pd-SCN, On removal of e.g. Cl^- by chromatography or addition of Ag⁺, the double cube re-forms.