The transition metal dihydrogen hypodiphosphate hydrates K2[Co(H2P2O6)2(H2O)2]·H2O (1), K2[Ni(H2P2O6)2(H2O)2]·H2O (2), K2[Cu(H2P2O6)2(H2O)2]·H2O (3) and K2[Zn(H2P2O6)2(H2O)2]·H2O (4) were synthesized and characterized by single crystal structure determination. The compounds 1–4 crystallize isotypic in the monoclinic space group C2/m (no. 12) with two formula units in the unit cell. The crystal structure is built up by [H2P2O6]2– units in an eclipsed conformation, by the corresponding transition metal, potassium cations, and water molecules. The eclipsed conformation of the [H2P2O6]2– has not been previously observed in none of known hypodiphosphates(IV) analyzed via X-ray diffraction. However, its proposed based on spectroscopic methods. FT-IR/FIR and FT-Raman spectra of the crystalline salts were recorded and the thermal behavior of the compounds was investigated.