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 (dmppyH(2) = 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( parallel). However, the values of the parameter tau 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( parallel) values. It is evident that out-of-plane distortions and ligand strain make calculation of tau inconsistent in certain sets of five-coordinate Cu(II) complexes. Structure-spectroscopy correlation involving tau is not feasible in such cases.