The earlier reported structure of the title compound,disodium tetracyanonickelate(II) trihydrate,Na 2 Ni(CN) 4.3H 2O, has been found to be incorrect andhas now been redetermined. The charge distribution hasbeen determined by multipole reÆnements againstsingle-crystal X-ray diffraction data. In the reÆnementbased on 30 K data a comparison was made between theresults obtained using hydrogen positions and displace-ment parameters from X-ray diffraction with those usingthe values determined by neutron diffraction. The spindensity was investigated by polarized neutron diffrac-tion at 1.6 K. Crystal data: at T = 30 K: a = 7.278 (4), b =8.856 (5), c = 15.131 (8) A, = 89.32 (5), = 87.39 (4),= 83.61 (4); at 295 K: a = 7.392 (4), b = 8.895 (4), c =15.115 (8) A, = 89.12 (2), = 87.46 (2), = 84.54 (2).The structure contains practically square planarNi(CN) 2ˇ4 ions, which are stacked on top of each otherin almost linear chains along the a direction. Theseparation between the Ni(CN) 2ˇ4 planes is rather large,with Ni–Ni distances around 3.7 A. The six crystal-lographically independent water molecules are eachcoordinated to two sodium ions, approximately in thetetrahedral (lone-pair) directions, and the polarizinginØuence of these sodium ions also appears to bereØected in the deformation density in the lone-pairplane. The charge density based on the deformationfunctions of all atoms in the structure is compared withthe individual densities calculated from the deformationfunctions of only nickel or the separate water molecules.In this way the effects of simple superposition of theindividual densities have been studied. In the planarNi(CN) 2ˇ4 ion the individual deformation density ofnickel is in qualitative agreement with that expectedfrom crystal-Æeld theory. As the repulsion from theelectrons is much weaker perpendicular to the Ni(CN) 2ˇ4plane than within this plane, the deformation density isconsiderably larger in the perpendicular direction.However, the largest maxima in the individual deforma-tion density around nickel are not found precisely in theplanes deÆned by nickel and the four cyanide ligands orin the perpendicular direction just mentioned, whichillustrates that it is necessary to consider the crystal Æelddue to the whole crystalline environment.