Lead ring-stacking

The packing of the arsenic or antimony rings in the crystals leads to stacks of rings. Structural data are given in Table 1. It is remarkable that the shortest intermolecular contacts between antimony atoms are considerably shorter than between arsenic atoms. With respect to intermolecular association in the solid state, however, (PhSb)g is a borderline case. The contact distances are close to the van der Waals distances between antimony atoms (440 pm). For the crystallization of the phenylantimony hexamer, the presence of cyclic organic molecules that are incorporated in the solid plays an important role. 

In both forms, the molecules were found to be composed of two nearly planar parts (the aromatic ring system and the allylic side chain) that are almost perpendicular to each other. The crystal packing in both forms is determined primarily by the 7t-interactions between aromatic rings, which leads to stacking forces and van der Waals forces. Only one intermolecular hydrogen bond was found in each structure, forming... 

Figure 10. Transformation of VFI framework simultaneously in the three possible directions leading to stacking disorders and ring misalignment.

Reaction of [Rh(/z-Cl)(CO)2]2 with sodium pyrazolate leads to 206 (85CJC699). The Rh2N2Cl ring has the envelope conformation. The rhodium atom has distorted square-planar coordination. The molecules in the crystalline lattice form onedimensional stacking units with alternating rhodium atoms in the binuclear units, intermolecularly interacting in a zigzag chain. 

This bistability is favoured by structures, which will adopt n—stacked motifs, i.e. lamellar molecules. In addition the tendency to adopt the planar structure is enforced by the presence of electronegative heteroatoms (currently restricted to N), which lead to a propensity of in-plane electrostatic S- N contacts between rings. These may comprise heterocyclic S- N contacts or S- N contacts to other functional groups such as the pyrazine or cyano nitrogen atoms. 

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