The electronic structure of copper in the polymer potassium bis(carbonato)cuprate(II) as determined from precise X-ray diffraction data

Abstract

The crystal structure of the polymer potassium bis(carbonato)cuprate(II), K₂[Cu(CO₃)₂], has been investigated in terms of the electronic structure of the copper atom. Single-crystal X-ray diffraction data with (sinθ)/λ⩽ 10.8 nm^–1, which were previously used in a 3σ(I) refinement of the crystal structure, were employed. Crystals are orthorhombic, space group Fdd2, Z= 8, a= 1.142 5(3), b= 1.765 8(4), c= 0.615 4(2) nm. All 1 693 independent reflections were used in a least-squares refinement on intensity values. Final agreement indices were R 0.029 and χ 2.027. The copper atom is in an essentially square-planar environment of four oxygen atoms from the bridging carbonato-groups [Cu–O 192.9(1) and 193.3(1) pm]. The final difference-Fourier synthesis showed non-centrosymmetric areas of positive and negative deformation charge density on opposite sides of the CuO₄‘plane.’ These regions of charge were included in the scattering model as spherical scatterers, and their populations refined to ±0.31 (4) electrons at fixed axial positions 55 pm from the copper atom. The dipole moment on the copper atom, parallel to the c crystal axis, is of magnitude 0.5(1)× 10^–29 C m. The symmetry of these deformation density features does not follow the local (bonded) square-planer symmetry of the copper atom environment. Two Cu ⋯ CO contacts of ca. 280 pm, which occur in only one axial direction, seem to be involved in the polarisation of the valence electrons of the copper atom. The non-centrosymmetry of these deformation density features precludes their modelling in terms of 3d or 4s atomic orbital populations.