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.