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+