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^-

Abstract

The kinetics of conversion of an edge-linked double cube, in this case [{Mo(3)PdS(4)(H(2)O)(9)}(2)](8+), to the corresponding single cube [Mo(3)(PdX)S(4)(H(2)O)(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 degrees C are as follows: CO, 1.11; PTA, 27.8; P(C(6)H(4)SO(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 K(1)/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 [Mo(3)FeS(4)(H(2)O)(10)](4+), which was selected because substitution at the Fe is also fast and there is no known double-cube formation. For both Mo(3)Pd and Mo(3)Fe the latter two stages can be explained by substitution at nonidentical (two alpha and one beta) H(2)O's on each Mo or by the presence of mixed-valent Mo(III)(2)Mo(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.