Retention of -4 Symmetry in Compounds Containing MAr4 Molecules and Ions

A search of the Cambridge Structural Database shows that MAr4 molecules usually retain the low-energy \overline 4 conformation in the crystal. MAr^{q}_{4} ions also usually occupy sites if the counterion can conform to \overline 4 (or higher) symmetry. The ability of the aryl groups to fit well around an empty \overline 4 site is crucial to the symmetry retention. Molecules (and ions) in the tetragonal structures pack in columns that parallel the c axis; the columnar cross sections are approximately square. Aryl groups in adjacent columns can interleave to form a sort of herringbone arrangement. The tetragonal packing of columns is sufficiently favorable that it sometimes occurs even if the molecules must be disordered. The columnar motif is also found in some lower-symmetry structures of MAr4 molecules and MAr^{q}_{4} ions; these structures may be pseudosymmetric.