Electronic Structure and Properties of Bunz' Polymers

The Bunz polymers consist of n-membered rings of carbon atoms connected via n mono- or diethynyl units (m = 1, 2 respectively) to other such rings. Coordinated to the three-, four-, and five-membered rings are ML<INF>n</INF><INF></INF> units (L = CO, Cp) which lead to 18-electron rule stability at each metal−organic center. The electronic band structures of the metal-free (often metallic) and metal-complexed (always insulating) materials are derived, and it is shown how the insulating polymer−M(CO)<INF>3</INF> systems may be converted to systems with small bandgaps by the loss of a CO group. Depending on the width of the conduction band, either small bandgap semiconductors, ferromagnetic metals or antiferromagnetic insulators are predicted. The properties of C<INF>6</INF>ML<INF>x</INF><INF></INF> polymers are also investigated (ML<INF>x</INF><INF></INF> = NO, CO, C<INF>3</INF>H<INF>3</INF>⁺). These consist of an ML<INF>x</INF><INF></INF> unit attached to a graphite sheet. Bending the MNO group converts the linear three-electron donor “NO⁺” unit into an “NO^-” moiety and generation of a metal. By construction of polymers with specific arrangements of M(CO)<INF>3</INF> and MCp groups, interesting patterns of metallic and insulating units may be envisaged at the atomic level.