Investigation of the aqueous coordination chemistry for citrate and vanadium(V) resulted in the isolation and characterization of K-2[V(O)2(C6H6O7)](2) . 4H(2)O (1) Complex 1 represents the first model of the tricarboxylic acid coordinated to the heteroatom found in the cofactor of nitrogenase and may be relevant to other physiological forms of metabolized vanadium(V). 1 was characterized by elemental analysis, IR, UV-vis, and V-51 NMR spectroscopy, and X-ray crystallography. Complex 1 crystallizes in the space group P2(1)/n (No. 14) with a 9.3205(8) Angstrom, b = 11.739(1) Angstrom, c = 11.913(1) Angstrom, beta = 111.65(1)degrees, V = 1211.4(4) Angstrom(3), and Z = 4. Full-matrix least-squares refinement resulted in residuals of R = 0.041 and R(w) = 0.049. The citrate ligand displays a unique bidentate coordination to the vanadium via the bridging hydroxyl group and a unidentate carboxylate group. The structure is best described as a dimer of two five-coordinate vanadium centers doubly bridged by hydroxyl oxygen atoms.