Dimeric Dioxomolybdenum(VI) and Oxomolybdenum(V) Complexes with Citrate at Very Low pH and Neutral Conditions

A novel dimeric dioxomolybdenum(VI) citrate complex, K[(MoO2)2(OH)(H2cit)2]·4H2O (1), with weak coordination of β-carboxylic acid
groups and the first structural example of an oxomolybdenum(V)
citrate complex, (NH4)6[Mo2O4(cit)2]·3H2O (2) (H4cit = 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 β-carboxyl and β-carboxylic acid
groups [2.621(9) Å]. 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 13C
NMR observations in solution.