The square-planar to flattened-tetrahedral CuX₄^2- (X = Cl, Br) structural phase transition in 1,2,6-trimethylpyridinium salts

Each (1,2,6-trimethylpyridinium)₂CuX₄ (X = Cl, Br) compound exhibits a structural phase transition in which the isolated complex anion geometry transforms from square-planar to flattened-tetrahedral (at 323 K for Cl, 225 K for Br), contrary to traditional expectations that such transitions only occur in chloride salts (as green-to-yellow thermochromism) in the presence of strong N-H hydrogen bonding.