EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE, CRYSTAL-STRUCTURES AT 295-KAND 173-K, AND ELECTRON-PARAMAGNETIC-RESONANCE AND ELECTRONIC-SPECTRAOF BIS[TRIS(2-PYRIDYL)METHANE]COPPER(II) DINITRATE

The crystal structure of bis[tris(2-pyridyl)methane]copper(II) dinitra te, [Cu{(C5H4N)3CH}2][NO3]2 has been determined. At 295 K the Cu atom lies on a special position so that all six Cu-N bonds are crystallogra phically equivalent [Cu-N 2.103(4) angstrom]. The structure at 173 K i s very similar [Cu-N 2.095(3) angstrom]. However, the electronic spect rum suggests that the Cu2+ ion experiences a ligand field of tetragona l symmetry. This has been confirmed by the EXAFS of the compound, whic h shows four nitrogen atoms at 2.04 angstrom and two at 2.25 angstrom from the copper. The apparent trigonal symmetry revealed by the X-ray analysis is thus due to disorder of the long and short Cu-N bonds abou t the three-fold axis. The EPR spectrum shows an isotropic signal at 2 95 K, but a signal characteristic of a tetragonally elongated octahedr al complex at 150 K. This suggests that the directions of the long and short bonds interchange rapidly on the EPR time-scale at room tempera ture, but that the complexes become frozen into particular orientation s on cooling.