ELECTRON-DENSITY DISTRIBUTION IN A CRYSTAL OF DIPOTASSIUM TETRAFLUORONICKELATE, K2NIF4

Dipotassium tetrafluoronickelate(II), K2NiF4, M(r) = 212.90, tetragonal, I4/mmm, a = 4.0130 (6), c = 13.088(2) angstrom, V = 210.78(4) angstrom3, Z = 2, T = 300 K, D(x) = 3.36 Mg m-3, Mo Kalpha, lambda = 0.7107 angstrom, mu = 656 mm-1, F(000) = 204, final R = 0.018 for 698 unique reflections [I>2sigma(I)]. Charge asphericity around the Ni atom caused by the splitting of 3d orbitals is clearly observed in the deformation density. Although the exact site symmetry at the Ni atom is D4h, the local geometry around Ni is practically O(h) with Ni-F distances of 2.0065 (3) and 2.0062 (8) angstrom. The d-orbital occupancies derive the multipole coefficients are in accordance with the prediction of simple crystal-field theory. The splitting of the d orbitals in D4h is e(g), b2g, (from t2g orbitals in O(h)), and b1g, (from e(g) in O(h)). The occupancies of all these orbitals are nearly equal with e(g) the largest (1.57), b1g and b2g the second largest (1.54), and a1g the smallest (1.51). Although the differences in occupancies are small, asphericity around Ni can be observed in the deformation-density distribution. Positive deformation density corresponding to e(g) (d(xz), d(yz)) orbitals, negative troughs corresponding to an a1g (d(z2)) orbital and slightly negative density corresponding to b1g (d(x2-y2)) and b2g (d(xy)) orbitals are manifested around the Ni atom.