The deformation of Di-mu-halide dinuclear five-coordinate copper(II) complexes in the crystalline state

The coordination geometries of 154 dinuclear di-mu-chloro, di-mu-bromo and di-mu-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 C-2, 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 reversible arrow 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 C-2 symmetry along the apical bond of a SQP, typical for the TBP reversible arrow SQP Berry path, are analyzed. A progressive reduction of the tetragonal elongation of the Cu-D apical (SQP) bond length along the SQP reversible arrow 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.