Chemical and structural data of numerous bis(α,β-dionedioximato)metal(II) compounds are summarized. All of them crystallize in columns but principally two different kinds of molecular arrangements occur in the solids. In one phase the molecular planes are inclined to the direction of the linear metal chains with an angle different from 90°. This allows only indirect interactions between the metal ions via the ligand. (“M—L—M” stacking.) The other modification consists of molecules with their planes perpendicular to the M—M-chains. This form allows direct metal-metal contacts (“M—M” modification). Depending on a few molecular parameters a “M—L—M” or a “M—M” stacking is obtained upon crystallization. Since for those compounds which could be isolated in both modifications the M—L—M form has the higher density it is concluded that only stronger M—M interactions stabilize the less dense M—M forms.A wide range of metal-metal separations with a lower limit of 3.15 Å in mixed valence systems are found in different “M—M” compounds. In any case the intrachain metal-metal distances are reduced considerably upon oxidation of the bivalent complex molecules. The influence of “electronic” and “sterical” parameters of the complex molecules on the intermolecular metal interactions and on the type of columns in the solid state is discussed.
