The formation of vanadium(III) complexes with nuclearity greater than two is believed to occur in aqueous solution on the basis of potentiometric, electrochemical, and/or UV-vis spectroscopy titration measurements, although structural evidence for this is limited. Upon the addition of 1-2 equiv of acetate, propionate, chloroacetate, trifluoroacetate, or bromoacetate to an aqueous, acidic solution of vanadium(III), trinuclear and tetranuclear complexes are formed. The structures of [V-4(mu-OH)(4)(mu-OOCCF3)(4)(OH2)(8)]Cl-4 center dot 7.5H(2)O (1), [V-4(mu-OH)(4)(mu-OOCCH3)(4)(OH2)(8)]Cl-4 center dot CH3COOH center dot 12H(2)O (2), [V-4(mu-OH)(4)(mu-OOCCH3)(4)(OH2)(8)]Cl-4 center dot 3H(2)O (3), [V-3(mu(3)-O)(mu-OOCCH2Br)(6)(OH2)(3)]CF3SO3 center dot H2O (4), [V-3(mu(3)-O)(mu-OOCCH2CH3)(6)(OH2)(3)]Cl center dot 2H(2)O (5), [V-3(mu(3)-O)(mu-OOCCH3)(6)(OH2)(3)]Cl center dot 3.5H(2)O (6), and [V-3(mu(3)-O)(mu-OOCCH2Cl)(6)(OH2)(3)]CF3SO3 center dot H2O (7) have been determined by X-ray diffraction. Importantly, electrospray mass spectrometry and H-1 NMR measurements suggest that these complexes are not purely solid-state phenomena but are also present in solution. For the vanadium(III)/acetate and vanadium(III)/propionate systems, two paramagnetic H-1 NMR signals corresponding to two distinct complexes (species A and B) are observed in the 40-55 ppm region for 0.20 mol equiv of acetate or propionate, at pD 3.44. No corresponding signals are observed for the vanadium(III)/bromoacetate and vanadium(III)/chloroacetate systems under the same conditions or for the vanadium(III)/ trifluoroacetate system using F-19 NMR spectroscopy. UV-vis spectra suggest that species B are structurally analogous for the vanadium(III)/acetate and vanadium(III)/propionate systems, whereas structurally different complexes are the major species for the other systems. Diffusion coefficients of species B for the vanadium(III)/acetate and vanadium(III)/propionate systems determined by pulsed-field-gradient spin-echo NMR spectroscopy measurements are (3.0 +/- 0.1) x 10(-6) and (3.23 +/- 0.01) x 10(-6) cm(2) s(-1), respectively, and are most consistent with species B being trimeric, rather than tetranuclear, complexes.