A novel dimeric dioxomolybdenum(VI) citrate complex, K[(MoO2)(2)(OH)(H(2)cit)(2)]center dot 4H(2)O (1), with weak coordination of beta-carboxylic acid groups and the first structural example of an oxomolybdenum(V) citrate complex, (NH4)(6)[Mo2O4(cit)(2)]center dot 3H(2)O (2) (H(4)cit = citric acid), are isolated in a very acidic solution (pH 0.5-1.0) and neutral conditions (pH 7.0-8.0), respectively. Complex 1 displays strong double hydrogen bonds through beta-carboxyl and beta-carboxylic acid groups [2.621(9) angstrom]. Transformations of the dimeric molybdenum(VI) citrate show that protonation of a carboxyl group will weaken the coordination of molybdenum(VI) citrate. There are obvious dissociations of molybdenum(VI/V) citrate complexes based on C-13 NMR observations in solution.