Chiral recognition among tris(diimine)-metal complexes, 6 - Racemic compound formation versus conglomerate formation with [M(bpy)(3)](PF6)(2) (M = Ni, Zn, Ru); Molecular and crystal structures

modifications in molecular size and shape induced by variation of the centr al atom drastically alter the crystallization behaviour of the hexafluoroph osphate salts of [M(bpy)(3)](2+). With M = Ru, racemic solutions crystalliz e as true racemates in the beta -type [P (3) over bar c1, a = 10.6453(5), c = 16.2987(9) Angstrom, Z = 2], while for M = Zn the racemic structure cann ot be obtained, as spontaneous resolution {gamma -type [P3(1), a = 10.3873( 5), c = 26.1309(16) Angstrom, Z = 3]} is the only crystallization path obse rved. For [Ni(bpy)(3)](PF6)(2), crystals of both structural types can be ob tained concomitantly from the same crystallization batch. The gamma -phase is systematically twinned by merohedry. The dramatic and unpredictable chan ge in the packing pattern of the very similar building blocks within this s eries arises from a delicate balance between the intermolecular forces. Dis tinct intermolecular interactions with different directionalities can be id entified in the two structure types. Through careful analysis of the crysta l and molecular structures, a trend with respect to molecular size and shap e on going from [Ru(bpy)(3)](PF6)(2) to [Zn(bpy)(3)](PF6)(2) can be establi shed. Even though one might speculate that increasing the size of the molec ular cation should destabilize the beta -type structure relative to the gam ma -type, no conclusive rationalization based solely on structural paramete rs can be given as regards why the relative order of the two local minima o n the energy hypersurface should become inverted.