KINETICS OF 1/1 THIOCYANATE (AND CHLORIDE) SUBSTITUTION AT MOLYBDENUM ON THE CUBOIDAL CLUSTERS [MO4S4(H2O)12]4+ AND [MO4S4(H2O)12]5+ IN AQUEOUS-SOLUTION

Substitution studies at 25-degrees-C, I = 2.00 M (LiClO4), on the title cuboidal Mo/S aqua ions are reported for the first time. Two consecutive reaction steps with large and small absorbance changes, respectively, are identified for the NCS- substitution on the MoIII4 ion [Mo4S4(H2O)12]4+. With thiocyanate in large >10-fold excess the first step corresponds to equilibrium kinetics, k(eq) = k(f)([NCS-]/4) + k(b), and the second (independent of [NCS-]) is assigned as isomerization, k(isom) = 3.6 X 10(-5) s-1. Rate constants for the first stage with cube in excess differ from those with NCS- in excess by a statistical factor of 4, corresponding to the number of equivalent Mo's, and forward and back rate constants are k(f) = 1.95 M-1 s-1 and k(b) = 1.44 X 10(-3) s-1, respectively. Two concurrent [NCS-]-dependent reactions are observed in the overall slower substitution on [Mo4S4(H2O)12]5+. For the first a statistical factor of 3 applies, and the rate constant k1f = 0.116 M-1 s-1 is assigned to substitution at each of three Mo(III)'s. For the second there is no statistical factor, and k2f = 0.0166 M-1 s-1 is assigned to substitution at the single Mo(IV). No isomerization step was detected for [Mo4S4(H2O)12]5+. Kinetic studies therefore provide the first evidence that the electron distribution approximates to MoIII3MoIV in the case of [Mo4S4(H2O)12]5+. The magnitude of the various rate constants is considered. For both the 4+ and 5+ cubes no variation of rate constants with [H+] was observed over the range 0.40-2.00 M. Whereas no absorbance changes were observed for [Mo4S4(H2O)12]4+ with Cl- up to 1.5 M, small absorbance changes for the Cl- anation of [Mo4S4(H2O)12]5+ gave a single detectable rate constant (9.7 x 10(-3) M-1 s-1) and a ratio k(NCS)/k(Cl) consistent with an Id mechanism. The formation constant for the reaction with Cl- (1.9 M-1) is approximately 10(3) less than that with NCS-.