Structure−Spectroscopy Correlation in Distorted Five-Coordinate Cu(II) Complexes:  A Case Study with a Set of Closely Related Copper Complexes of Pyridine-2,6-dicarboxamide Ligands

Eight Cu(II) complexes with the [Cu(dmppy)] moiety (dmppyH₂ = tridentate ligand N,N‘-dimethylpyridine-2,6-dicarboxamide; H's are dissociable amide protons) and ligands like pyridine, water, N-methylimidazole, substituted and unsubstituted o-phenanthroline, and bipyridine have been isolated and structurally characterized. The basal angles of these structurally related five-coordinate Cu(II) complexes (and two previously reported ones) correlate well with the EPR hyperfine splitting parameter A_∥. However, the values of the parameter τ which provides a measure of the degree of square pyramid versus trigonal bipyramid geometry adopted by these complexes do not correlate linearly with the A_∥ values. It is evident that out-of-plane distortions and ligand strain make calculation of τ inconsistent in certain sets of five-coordinate Cu(II) complexes. Structure−spectroscopy correlation involving τ is not feasible in such cases.