KINETICS OF SUBSTITUTION OF H2O BY NCS- ON MU-SELENIDO INCOMPLETE CUBOIDAL MOIV3 CLUSTERS [MO3OXSE4-X(H2O)9]4+ AND ON [MO4SE4(H2O)12]5+

Rate constants (k(f)/M-1 s-1) have been determined at 25-degrees-C, I = 2.00 M (LiClO4 and in one case Lipts; pts = toluene-p-sulfonate) for the 1:1 substitution of H2O by NCS- at Mo on the trinuclear Mo(IV)3 cluster complexes [Mo3OxSe4-x(H2O)9]4+, x = 0-4. The dominant process is substitution of H2O at the trans mu-Se positions with rate constants for [Mo3(mu3-Se)(mu-Se)3(HO)9]4+ (480) > [Mo3(mu3-Se)(mu-O)(mu-Se)2(H2O)9]4+ (131 and 52) > [Mo3(mu-Se)(mu-O)2(mu-Se)(H2O)9]4+ (13.5 and 2.8) > [Mo3(mu3-Se)(mu-O)3(H2O)9]4+ (0.19), alongside the previously studied [Mo3(mu3-O)(mu-O)3(H2O)9]4+ (2.13) giving a 2530-fold spread of values at [H+] = I = 2.00M. The complexes with x = 1 and 2 have non-identical molybdenum centres according to the number of core Se2- and O2-ligands attached, and give biphasic kinetics. Statistical factors of 1, 2 and 3 corresponding to the number of identical centres are defined for the different reactions. The slowness of substitution at positions trans to the mu3-Se ligands is confirmed by studies on cuboidal [Mo4Se4(H2O)12]5+, which has all mu3-Se core ligands. Comparisons are made with results obtained for the [Mo3OxS4-x(H2O)9]4+ series. Labilising effects of electron-rich mu-Se2- > mu-S2- > mu-O2- at the trans H2O ligand are noted. In contrast replacement of the mu3-O of (Mo3O4(H2O)9]4+ by mu3-Se and mu3-S in turn produces 14- and 6-fold retardation effects. Substitution at the H2O ligands trans to mu-Se ligands is increased on decreasing [H+] consistent with the involvement of conjugate-base forms Mo3(OH)3+ (k2) alongside Mo-3(4+) (k1). The kinetics give H2O ligand acid dissociation constants K(aM) of 0.32 and 0.44 M for [Mo3Se4(H2O)9]4+ and (Mo3O3Se(H2O)9]4+ respectively. Similar patterns are observed for the aquation rate constants k(aq).