Ab Initio Configuration Interaction Calculation of the Exchange Coupling Constant in Hydroxo Doubly Bridged Cr(III) Dimers

Abstract

Ab initio difference-dedicated configuration interaction (DDCI) calculations have been carried out on the exchange coupling constant of the hydroxo doubly bridged Cr(III) complex [Cr2(μ-OH)2(NH3)8]4+, and the magneto-structural dependence on the bridge angle, Cr−O−Cr, and on the OH out-of-plane distortion have been analyzed. The complex is antiferromagnetic for all the geometries calculated. For the planar structure of the (CrOH)2 unit, the magnitude of the coupling constant calculated for a Cr−O−Cr bridge angle of 100°, −10.6 cm-1, is in good agreement with the experimental values found in complexes having almost planar structure of this unit and comparable bridge angles and external ligands such as ethylenediamine. When the geometry of the complex is defined according to the crystallographic data of [Cr2(μ-OH)2(NH3)8](S2O6)24H2O (I) and [Cr2(μ-OH)2(NH3)8] Cl44H2O (II), with OH out-of-plane distortions of 24 and 41°, respectively, the antiferromagnetic character decreases as the distortion increases. The DDCI calculated J, −6.1 cm-1 for the dithionate I geometry and −4.2 cm-1 for the chloride II one, are in very good agreement with experiment: −2.9 and −2.1 cm-1, respectively. The influences of the basis set and of the counterions are also discussed.