The Deformation of Di-μμ-halide Dinuclear Five-Coordinate Copper(II) Complexes in the Crystalline State

The coordination geometries of 154 dinuclear di--chloro, di--bromo and di--fluoro five-coordinate (4 + 1) copper(II) complexes have been analyzed by the structural correlation method. Two reference Cu^II polyhedra have been established: (i) a trigonal bipyramid (TBP) in which the equatorial bond lengths are equal, but with the symmetry at the metal often deformed from D_3h to C₂, and (ii) an elongated square pyramid (SQP), with either a pyramidally or tetrahedrally distorted base, in which the trans valence angles are equal. Six different paths for the SQP⇌TBP deformation of the Cu^II coordination have been established from three criteria: (a) the location of the bridging ligands (axial and equatorial or equatorial and equatorial in TBP); (b) the nature of the deformation of the SQP base plane (pyramidal or tetrahedral); (c) whether a bridging or nonbridging bond is elongated in a SQP. The causes of the angular distortions from C₂ symmetry along the apical bond of a SQP, typical for the TBP⇌SQP Berry path, are analyzed. A progressive reduction of the tetragonal elongation of the Cu—D apical (SQP) bond length along the SQP⇌TBP transformation path and trigonal equalization of the three equatorial Cu—D (TBP) bonds are observed. The dependence of the average Cu—Cu′ distance and Cu—X—Cu′ angles on the deformation path are also established.