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

The abrupt green-to-yellow thermochromism observed for certain A2CuCl4 compounds has long been attributed to the presence of strong N-H hydrogen bonding in the low-temperature phase that favors square-planar CuCl4(2-) (green in color) through reduction of ligand-ligand repulsion. Weakening of the hydrogen bonding at higher temperature results in the transformation to (far more common) flattened-tetrahedral geometry - the expected geometry for CuBr4(2-) complexes due to their greater ligand-ligand repulsion. The square-planar to flattened-tetrahedral transitions in (1,2,6-trimethylpyridinium)2CuX4 provide the first examples of this phase transformation in the absence of N-H hydrogen bonding and for a CuBr4(2-) complex.

[CuCl3(H2O)](-) complexes aggregated to form hydrate columns in methyl-substituted pyridinium or piperidinium salts.

1,2,3-Trimethylpyridinium aquatrichloridocuprate(II), (C(8)H(12)N)[CuCl(3)(H(2)O)], (I), 3,4-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (II), and 2,3-dimethylpyridinium aquatrichloridocuprate(II), (C(7)H(10)N)[CuCl(3)(H(2)O)], (III), exhibit the same fundamental structure, with (I) and (II) isomorphous and with the unit-cell constants of (III) similar to the reduced unit-cell constants of (I) and (II). The distorted square-planar [CuCl(3)(H(2)O)](-) complex [mirror symmetric in (I) and (II)] forms two semicoordinate Cu···Cl bonds to a neighboring complex to produce a dimer with 2/m symmetry [only inversion symmetry in (III)]. The semicoordinate Cu.