An instrumental experiment is presented in which eleven Cu(II) complexes are studied with electron paramagnetic resonance (EPR) spectroscopy. The EPR spectroscopy allows the characterization of the geometry and electronic structure of the copper complexes. Three common ligands, ethylenediamine (en), 1,10-phenanthroline (phen), and 2,2′-bipyridine (bpy), were used. Examination of the EPR spectra of the solid-state compounds demonstrates that [Cu(en) 2 (ClO 4 ) 2 ], [Cu(en) 2 (BF 4 ) 2 ], and [Cu(en) 2 (NO 3 ) 2 ] have a d x 2 -y 2 ground state and an elongated octahedral geometry; that [Cu(phen) 2 (H 2 O)](NO 3 ) 2, [Cu(phen) 2 (H 2 O)](BF 4 ) 2, and [Cu(bpy) 2 Cl]ClO 4 are characterized by a geometry intermediate between the square pyramid and the trigonal bipyramid and a ground state corresponding to the linear combination of the d x 2 -y 2 and d z 2 orbitals; and that [Cu(phen) 2 Cl]ClO 4, [Cu(phen) 2 Br]ClO 4, [Cu(bpy) 2 Br]ClO 4, [Cu(phen) 2 ]ClO 4, and [Cu(bpy) 2 I]ClO show a geometry close to the trigonal bipyramid and a ground state of d z 2 . A theoretical explanation of the results is presented.
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Giovanni Micera (2006) studied this question.