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The square-planar to flattened-tetrahedral CuX4(2-) (X = Cl, Br) structural phase transition in
Annette Kelley1, Sowjanya Nalla1, Marcus R Bond1
1Department of Chemistry, Southeast Missouri State University, One University Plaza, Cape Girardeau, Missouri 63701, USA.
Abstract:
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. These results suggest that the square-planar to flattened-tetrahedral transformation in CuX4(2-) systems may be more common than previously thought.
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