We have synthesized and structurally characterized three pyridylethylidene-functionalized diphosphonate-containing polyoxomolybdates, [{Mo(VI)O(3)}(2){Mo(V)(2)O(4)}{HO(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)](6-) (1), [{Mo(VI)(2)O(6)}(2){Mo(V)(2)O(4)}{O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)](8-) (2), and [{Mo(V)(2)O(4)(H(2)O)}(4){O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(4)](12-) (3). Polyanions 1-3 were prepared in a one-pot reaction of the dinuclear, dicationic {Mo(V)(2)O(4)(H(2)O)(6)}(2+) with 1-hydroxo-2-(3-pyridyl)ethylidenediphosphonate (Risedronic acid) in aqueous solution. Polyanions 1 and 2 are mixed-valent Mo(VI/V) species with open tetranuclear and hexanuclear structures, respectively, containing two diphosphonate groups. Polyanion 3 is a cyclic octanuclear structure based on four {Mo(V)(2)O(4)(H(2)O)} units and four diphosphonates. Polyanions 1 and 2 crystallized as guanidinium salts [C(NH(2))(3)](5)H[{Mo(VI)O(3)}(2){Mo(V)(2)O(4)}{HO(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)]·13H(2)O (1a) and [C(NH(2))(3)](6)H(2)[{Mo(VI)(2)O(6)}(2){Mo(V)(2)O(4)}{O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(2)]·10H(2)O (2a), whereas polyanion 3 crystallized as a mixed sodium-guanidinium salt, Na(8)[C(NH(2))(3)](4)[{Mo(V)(2)O(4)(H(2)O)}(4){O(3)PC(O)(CH(2)-3-C(5)NH(4))PO(3)}(4)]·8H(2)O (3a). The compounds were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. The formation of polyanions 1 and 3 is very sensitive to the pH value of the reaction solution, with exclusive formation of 1 above pH 7.4 and 3 below pH 6.6. Detailed solution studies by multinuclear NMR spectrometry were performed to study the equilibrium between these two compounds. Polyanion 2 was insoluble in all common solvents. Detailed computational studies on the solution phases of 1 and 3 indicated the stability of these polyanions in solution, in complete agreement with the experimental findings.