YAlC: A Bonding Chameleon with Heteropolyacetylene Features.

YAlC was prepared by a flux method. It crystallizes as a partially filled-up TlI structure, showing remarkable structural aspects at the border between Zintl phases and intermetallics. This novel ternary aluminide-carbide exhibits a unique one-dimensional multi-center bond and a polyacetylene-related aluminum carbide substructure. The different functionalities of aluminum and of yttrium are quite remarkable. While the latter behaves more like a trivalent ion, aluminum contributes considerably to covalent bonding with carbon. Still yttrium d states contribute, but hardly in a directed manner.