Neighbourstrand interactions in one-dimensional crystal orbital calculations on organometallic polymers-the tetrathiosquarato nickel(II) system

Abstract

The band structure of tetrathiosquarato nickel(II), TTSqNi 1 , is studied by means of crystal orbital (CO) calculations based on a semi-empirical INDO Hamiltonian under the inclusion of neighbourstrand interactions. The interchain coupling is determined in the framework of a self-consistent electrostatic field approximation where the band structure of the reference stack is influenced due to the potential of the neighbouring polymer backbones. Intrachain polarization and electrostatic Coulomb energies are considered in the present CO model. The dispersion of the reference strand as well as charge redistributions and charge shifts are investigated as a function of the number of the neighbouring chains considered in the tight-binding formalism, the mutual orientation of the various backbones in the “real solid” and the strength of the external field. Charge shifts, modifications in the “covalency” of the different bonds, alterations in the ε( k) curves and density of states distribution as response to the external field are discussed. It is shown that symmetry broken solutions (charge density waves, CDWs) are encountered in the CO formalism if the strength of the external potential exceeds a critical value. This charge condensation is however subject to all limitations given by the mean-field approximation. Necessary condition for CDW solutions are mutual displacements between the TTSqNi backbones in the direction on the stacking axis. General rules on the importance and the consequences of interstrand interactions in low-dimensional materials with organometallic fragments as molecular building blocks are formulated.