KINETIC-STUDIES ON THE TERMINAL-LIGAND-SUBSTITUTION REACTIONS OF ACETATE-BRIDGED TRINUCLEAR MOLYBDENUM AND TUNGSTEN CLUSTER COMPLEXES, [MO3(MU-3-CCH3)(MU-3-O)(MU-CH3COO)6(L)3]+ (L = H2O OR PYRIDINE) AND [W3(MU-3-O)(MU-CH3COO)6(H2O)3]2+

Three different reactions of trinuclear acetato-bridged molybdenum and tungsten cluster complexes have been studied by following the change in H-1 NMR spectra. Stepwise substitution reactions of solvent CD3OD for the terminal aqua ligands on [Mo3(mu-3-CCH3)(mu-3-O)(mu-CH3COO)6(H2O)3]+ (formal oxidation state of the Mo3 is (IV,IV,IV)) (-30.2 to -15.3-degrees-C) and [W3(mu-3-O)(mu-CH3COO)6(H2O)3]2+ (III,III,IV) (9.5-29.6-degrees-C) in CD3OD gave first-order rate constants (per metal ion) of the first steps of (9.4 +/- 1.5) x 10(-5) s-1 at -22.5-degrees-C (DELTA-H-double-ending-dagger = 93 +/- 11 kJ mol-1; DELTA-S-double-ending-dagger = +49 +/- 47 J K-1 mol-1) and (5.5 +/- 0.6) x 10(-5) s-1 at 24.5-degrees-C (DELTA-H-double-ending-dagger = 113 +/- 3 kJ mol-1; DELTA-S-double-ending-dagger = +56 +/- 9 J K-1 mol-1), respectively. A pyridine exchange reaction with py-d5 (0.5-2.6 M) for the new complex [Mo3(mu-3-CCH3)(mu-3-O)(mu-CH3COO)6(py)3]+ in CD3NO2 shows no [py-d5] dependence at 10-35-degrees-C. The first-order rate constant at 20.8-degrees-C is (8.6 +/- 1.0) x 10(-4) s-1 (DELTA-H-double-ending-dagger = 112 +/- 1 kJ mol-1; DELTA-S-double-ending-dagger = +77 +/- 5 J K-1 mol-1) (-0.3 to +34.6-degrees-C). Results for the two Mo3(mu-3-CCH3)(mu-3-O) complexes clearly indicate a strong dependence of the substitution rates on the type of leaving ligands even if the difference in solvents is taken into account. The CD3OD substitution rate constant is significantly larger than the corresponding value (1.1 x 10(-5) s-1 at 44.8-degrees-C per metal ion) for the bis(mu-3-oxo) complex [Mo3(mu-3-O)2(mu-CH3COO)6(H2O)3]2+ (IV,IV,IV), indicating a strong labilization effect of the mu-3-CCH3(3-) ligand. The rate constant for the W3 complex is at least 2 x 10(3) times larger than that ( < 5 x 10(-7) s-1 at 44.8-degrees-C) for the double-capped complex [W3(mu-3-O)2(mu-CH3COO)6(H2O)3]2+ (IV,IV,IV). A difference in the formal oxidation state should be responsible in this case. An I(d) or D mechanism is suggested for all the terminal ligand substitution reactions of these trinuclear cluster compounds.