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 tetracyanoquinodimethane (BTDMTTF-TCNQ, formally C10H8S6+. C12H4N4-) have been analyzed using 130 K single-crystal x-ray-diffraction data up to a resolution of(sin theta/lambda)(max) = 1.35 Angstrom(-1). A multipolar pseudoatom density model was fitted against the 5145 observed data with I > 3 sigma(I), [R(F) = 0.023, R-w(F) = 0.022, GoF = 0.91] optimizing the set of (n(1); alpha) parameters of the Slater-type sulfur radial 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-wave behavior appearing in this complex is in relation to the external sulfur hybridization. Furthermore, the topological characterization of the electron density shows the isolated character of the external sulfur 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 similar to most hydrogen bonds, showing a typical saddle point between the donor and the acceptor atoms.