Two new compounds based on Weakley-sandwich-type polyanion (WSP), H 2 [Mn(en) 2 ] 2 [Mn(en) 2 (H 2 O)] 2 [Mn 4 (H 2 O) 2 (PW 9 O 34 ) 2 ]·9H 2 O (1) and Na 2 [Co(en) 2 ] 2 [{Co(en) 2 (H 2 O)] 2 [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ]·9H 2 O (2) (en=ethylenediamine), have been synthesized by the hydrothermal reaction of the trivacant Keggin polyoxoanion [a-A-PW 9 O 34 ] 9− with transition metal ions and en, and structurally characterized by routine physical methods and single-crystal X-ray diffraction. Compound 1 represents a 1D chain structure made up of WSPs and [Mn(en) 2 ] 2+ ions. Compound 2 represents a 2D layer structure formed by the interconnection of WSPs, [Co(en) 2 ] 2+ and Na + ions. Compounds 1 and 2 represent the first example of high-dimensional construction based on WSPs and transition metal complexes (TMCs), in which the water molecules located on the featured tetra-metal unit are still remained, and exhibit the property of sorption/desorption of water molecules.