Stacked metal complexes: Structures and properties

Abstract

We discuss the structural aspects of crystals that contain molecular stacks and are comprised of two components, at least one of which is a planar transition-metal chelate complex. One of the components may be viewed as an electron donor, D, or its cation, D+p, the other an acceptor, A, or its anion, A−q. In these crystals either one or both components tend to form stacks, and we employ a classification scheme whose first major division is structural: crystals either contain stacks that integrate the D and A units, or the D and A units can be segregated with D or A stacking, or both. The second major division is physical. In an integrated stack crystal the metal-ligand complex is either neutral or ionic; in a segregated stack crystal it either has an integral oxidation state or a non-integral oxidation state. The non-integral oxidation state is sometimes referred to as “partial oxidation”, “mixed valence”, or “incomplete charge transfer”. The compounds discussed here exhibit a broad range of fascinating properties. Some of these compounds are primarily of structural interest, while others have unusual magnetic properties. Yet others, particularly those involving metallomacrocycles, exhibit high or unusual electrical conductivities. By classifying and comprehensively reviewing the structural data available, we hope to provide a firmer basis for improving the understanding of the relation between the physical properties and the structural features of these compounds.