Characterization of dimeric (carboxylato)copper(II) complexes by electron spin resonance spectra. Correlation of ESR parameters with singlet-triplet energy separations

Linear correlations with positive slopes have been found for the zero-field splitting parameters (D) and the anisotropic exchange parameters (D-ex) vs the singlet-triplet separations (-2J) for [Cu-2(O2CR)(4)(L)(2)] (R = F3C, FCH2, Cl3C, ClCH2, CH3, H, and C6H5CO) for a wide range of - 2J values, ca. 240-650 cm(-1). The values of D and D-ex for benzoylformate were found to be the highest ever studied. The quality of the Linear relations of -2J, D-ex and D with the square of the coefficient of 2p(x)(C) of the symmetric HOMO of the bridging carboxylate ions, coeff,(2) were found to be in the order, -2J vs coeff(2) > D-ex vs coeff(2) > D vs coeff(2). A roughly linear relationship with a negative slope was found for D vs -2J for a system, [Cu-2(O2CCMenPh3-n)(4)(L)(2)], in which the -2J values cover a relatively small range, ca. 320-370 cm(-1).