Polymeric coordination compounds based on Cr₂ⁿ⁺ paddle-wheel building blocks with non-carboxylate O,O-donor ligands chelating and bridging the Cr–Cr centers have been underexplored hitherto. This paper reports the synthesis and crystal structure of a new homo-valent chromium(II,II) compound, Na₃HCr₂(CO₃)₄·10H₂O (1). It has a two-dimensional structure in which the paddlewheel chromium(II,II) units of Cr₂(CO₃)₄⁴⁻ are cross-linked through the carbonate groups. The layers are stacked along the [100] direction, and Na ions fill the intra and interlayer spaces with a neighboring layer distance of about 11.3 Å. The investigation of the primary magnetic properties and theoretical studies with density functional theory (DFT) reveal the partial paramagnetic properties of compound 1 arising from the Boltzmann distribution between a ground state σ²π⁴δ² with S = 0 and a low-lying excited state σ²π⁴δδ* with S = 1 for the Cr₂(II,II) dimer. According to Raman spectra measurements combined with theoretical calculations, the two Raman bands in the small-wavenumber region at 335 and 297 cm⁻¹ were assigned to the vibration of the Cr–O bonds and the band at 371 cm⁻¹ was assigned to the stretching of the Cr–Cr quadruple bond.