The Zero-Field-Splitting Parameter D in Binuclear Copper(II) Carboxylates Is Negative

High-field, high-frequency electron paramagnetic resonance measurements performed on two binuclear copper complexes, [Cu(CH3COO)2]2(H2O)2 (1) and [Cu(CH3COO)2]2(pyrazine) (2), allowed a definitive determination of the sign of the zero-field-splitting parameter D. Contrary to literature reports, that parameter is negative in each complex.