The dimeric molybdenum(II) complex, Mo2(CF3COO)4, forms Mo2(CF3COO)4·(CH3CN)2 in acetonitrile, where solvent molecules are attached to positions trans to the metal–metal bond (apical positions). The rate of ligand exchange between Mo2(CF3COO)4 ((1.31–8.28)×10−2 M; M=mol dm−3) and CF3COONa ((1.31–4.56)×10−1 M) was measured in acetonitrile by the use of 19F NMR spectroscopy. The rate is independent of the total concentration of CF3COONa and is linearly dependent on that of Mo2(CF3COO)4. The first-order rate constant (kex) is (1.1±0.2)×104 s−1 (25 °C), and the activation parameters are ΔH\eweq=(8.2±0.8) kcal mol−1 and ΔS\eweq=(−15±3) K−1 mol−1. Under the given conditions the complex seems to exist as Mo2(CF3COO)4·CF3COO−, where the underlined ligand occupies one of the apical positions, and the following mechanism is proposed (solvent molecules are omitted; asterisks indicate the molecule substituting for the coordinated ligand).Mo2(CF3COO)4·CF3COO−+*CF3COO−\oversetfast\ightleftarrowsMo2(CF3COO)4·*CF3COO−+CF...