AQUEOUS-SOLUTION CHEMISTRY OF THE MO3PDS4 CUBE - SUBSTITUTION-REACTIONS AND THE DOUBLE TO SINGLE CUBE INTERCONVERSION INDUCED BY CO, 2 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 [Mo-3(Pd X)S-4(H2O)(9)](4+), has been studied for the first time. Reaction is i nduced by six reagents X = CO, two water-soluble phosphines, Cl-, Br-, and NCS-, which complex at the tetrahedral Pd. The first stage of rea ction 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 inte nse enough for stopped-flow monitoring with reactants at less than or equal to 1 mh, rate constants 10(-5) k/M(-1) s(-1) at 25 degrees C are as follows: CO, 1.11; PTA, 27.8; P(C6H4SO3)3(3)-, 9.6; at I = 2.00 (L i(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 const ants K-l/M(-1) were determined, Cl-(490), Br- (8040), and NCS- (630), by UV-vis spectrophotometry. Two subsequent kinetic stages are assigne d to substitution at the Mo's. Similar behavior is observed for [Mo3Fe S4(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 n onidentical (two alpha and one beta) H2O's on each Mo or by the presen ce of mixed-valent (Mo2MoIV)-Mo-III forms which are sufficiently long- lived to give a kinetic discrimination. In the case of NCS- an additio nal step, 0.015 s(-1), independent of [NCS-] is assigned to the isomer ization Pd-NCS--> Pd-SCN. On removal of e.g. Cl- by chromatography or addition of Ag+, the double cube re-forms.