BONDING PROPERTIES OF Cu(II)-IMIDAZOLE CHROMOPHORES: ELECTRONIC AND MOLECULAR STRUCTURE OF Cu(IMIDAZOLE)4(CF3SO3)2

The crystal and molecular structure of [Cu(imidazole)4(CF3SO3)2] has been determined using three-dimensional X-ray diffraction data. The complex crystallizes in the monoclinic space group P21/n with Z = 2, a = 9.116(4), b = 9.560(1), c = 14.664(5) Å, and β = 106.19(3)°. Least-squares refinement of the structure yields a final R value of 0.041 and Rw of 0.052 for 1607 independent reflections at 298 K. The copper(II) ion is coordinated centrosymmetrically by four imidazole ligands forming an equatorial plane, mean Cu-N 2.010 Å, and by two trifluoromethanesulfonate ions at the axial sites, Cu-O 2.593(5) Å. The dihedral angles between the imidazoles and the CuN4 plane are 88.6° and 59.4°. Gaussian analyses of the electronic spectra of four tetrakis(imidazole)copper(II) complexes having O-donor counter ions yield the d orbital sequences as d x2−y 2 ≫ d z 2 > d xy > d xz ≧ d yz, depending on the orientations of the imidazole nuclei. Energy differences between the antibonding d z 2 and non-bonding d xy orbital correlate well with the Cu-O bond lengths and this supports the sequences of the d orbitals.