ELECTRON-DENSITY STUDY OF THE ONE-DIMENSIONAL ORGANIC METAL BIS(THIODIMETHYLENE)-TETRATHIAFULVALENE TETRACYANOQUINODIMETHANE

The crystal structure, thermal vibrations, and electron density of the charge-transfer complex bis(thiodimethylene)-tetrathiafulvalene tetra cyanoquinodimethane (BTDMTTF-TCNQ, formally C10H8S6+. C12H4N4-) have b een analyzed using 130 K single-crystal x-ray-diffraction data up to a resolution of(sin theta/lambda)(max) = 1.35 Angstrom(-1). A multipola r pseudoatom density model was fitted against the 5145 observed data w ith I > 3 sigma(I), [R(F) = 0.023, R-w(F) = 0.022, GoF = 0.91] optimiz ing the set of (n(1); alpha) parameters of the Slater-type sulfur radi al function. This electron-density study allows us to understand that the charge transfer between ions (approximate to 0.7 e) involves only the external sulfur atom in the cation and that the charge-density-wav e behavior appearing in this complex is in relation to the external su lfur hybridization. Furthermore, the topological characterization of t he electron density shows the isolated character of the external sulfu r atom in relation to the rest of the cation, and explains that charge transfer occurs via the interaction between the external sulfur atoms and the triple bonds C=N of the anion, concentrating the transferred charge mainly on nitrogens. The electrostatic potential calculated in the region of the charge transfer cation-anion is topologically simila r to most hydrogen bonds, showing a typical saddle point between the d onor and the acceptor atoms.