The crystal structures of the title compounds, (C7H9N)3CuX, X = Cl , Br, I [(1), (2), (3)], have been determined by single-crystal X-ray methods at 295 K, being refined to residuals of 0.022, 0.053, 0.047 for 151, 312, 720 independent 'observed' reflections respectively. The three structures containing 3,5-dimethylpyridine have an interesting relationship in symmetry: (1) is rhombohedral , R3m, a
9.050(3)Ǻ, α 110.89(3)°, Z 1; (2) is rhombohedral, R3c, a 10.328(5)Ǻ, α 95.60(4), Z 2; and (3), derived from (2), is monoclinic, Cc, a 13.233(6), b 15.083(7), c 11.491(6)Ǻ, β 98.27(4)°, Z 4. The distances Cu- Cl , Br, I are 2.412(9), 2.51(1), 2.683(3)Ǻ, with Cu-N 2.08(1), 2.02(1), 1.98(2)- 2.15(4)Ǻ respectively. In (1), the symmetry of the molecule, containing pseudo- tetrahedrally coordinated copper, N3CuX, is a full 3 m, with the ligands lying in mirror planes containing the 3 axis; in (2), the ligands rotate about the Cu-N bonds so that the molecular symmetry is 3; and in (3) further distortion of this kind causes total loss of crystallographically imposed symmetry. This descent in symmetry is the converse of that observed in the rhombohedral series [(C6H7N)3CuX] (C6H7N = 3-methylpyridine), and the possible underlying reasons are explored in terms of intra- and inter-molecular X...H interactions.