The title polymer, [Mn(C2O4)(H2O)2]n, was synthesized by the reaction of manganese(II) nitrate tetrahydrate with a protontransfer compound, (pipzH2)(ox) (where oxH2 is oxalic acid and pipz is piperazine), in aqueous solution. The structure features a six-coordinate Mn2+ ion located on a twofold rotation axis, with a distorted octahedral geometry [Mn—O = 2.1728 (8)–2.1914 (9) A ˚ ]. The bond angles show that the two coordinated water molecules are arranged trans. One-dimensional linear chains are formed through bis-bidentate oxalate ligands bridging the MnII ions. These structural units are held together by O—H O hydrogen bonds. A neutron diffraction study of [Mn(-ox)(D2O)2]n, using powdered samples, has been published recently in the same space group [Sledzinska, Murasik & Fischer (1987). J. Phys. Solid State Phys. 20, 2247– 2259]. However, it should be noted that an orthorhombic polymorph for the title polymer is also known [Huizing, van Hal, Kwestroo, Langereis & van Loosdregt (1977). Mater. Res. Bull. 12, 605–611; Lethbridge, Congreve, Esslemont, Slawin & Lightfoot (2003). J. Solid State Chem. 172, 212–218]. The present single-crystal X-ray study deals with the monoclinic polymorph. This phase is isostructural with the CoII analogue.
