Room-temperature C-13 NMR spectra of the d(1)-d(1) dimer [Mo2O3{(S2CN)-C-13(CH(2)Ph)(2)}(4)] (1) exhibit a broad singlet for the thiocarboxylate ligand. The molecule is fluxional, and in the low-temperature limit this singlet is replaced by two assymetrical doublets (with components at delta 207.4, 207.2 and delta 200.7, 199.7) assigned to the thiocarboxylate ligands with one S trans to the bridging oxo group and one S trans to the terminal oxo group respectively of the syn and the anti isomers of 1. The barrier to exchange has been determined between 320 and 335 K from the width of the resonance in the fast exchange region, and the small entropic contribution to the barrier (Delta H+ = 11.9 +/- 0.4 kcal mol(-1), Delta S+ = -2.0 +/- 0.6 cal K-1 mol(-1)) argues in favor of an intramolecular mechanism for the exchange reaction in which the thiocarbamate ligands are rendered equivalent by exchange of bridge and terminal oxo groups through a transition state in which the linear oxo bridge has been replaced by two bent oxo bridges. This interpretation is supported by the observation that the related xanthate complexes [Mo2O3(S(2)(13)COEt)(4)] (4) and [Mo2O3((S2COi)-C-13-PR)(4)] (5) participate in similar exchange processes with Delta H+ = 11.1 +/- 0.3 and 12.9 +/- 0.4 kcal mol(-1) and Delta S+ = -3.9 +/- 1.2 and 1.9 +/- 0.6 cal K-1 mol(-1), respectively. The small entropic contributions are again consistent with an intramolecular mechanism, and the similarity in enthalpic parameters for 1, 4, and 5 argues against a mechanism involving dissociation of the [(Mo2O3)-O-V](4+) dimers into [(MoO)-O-VI](2+) and [(MoO)-O-IV](2+) containing monomers since such disproportionations are ligand sensitive. Single crystal diffraction studies have established that 1 (monoclinic space group P2(1)/n, with a = 11.604(4) Angstrom, b = 21.81(2) Angstrom, c = 13.85(1) Angstrom, beta = 99.90(2)degrees, Z = 2, and R = 4.37%) and 5 (monoclinic space group C2/c, with a = 25.508(5) Angstrom, b = 9.659(2) Angstrom, c = 14.986(4) Angstrom, beta = 101.96(2)degrees, Z = 4, and R = 5.52%) have anti and syn orientations respectively for the terminal oxo groups in the solid state. All three complexes exist as a mixture of syn and anti isomers in solution, establishing that there is only a small free energy difference between these geometries.