Optical transitions in D_4h Copper chloride complexes : AN MS-Xα investigation

D.C.I.T.T.Y.M., Sección Ciencia de Materiales, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain

Abstract

The crystal-field (CF) and charge transfer (CT) spectrum of the square-planar CuCl₄^2- complex as well as the ones relevant to the elongated and compressed CuCl₆^4- complexes have been investigated through the MS-Xα methodology. The calculations performed for different values of the equatorial and axial distances R_eq and R_ax, reproduce well the experimental values found for CuCl₄^2- (R_eq=226.5 pm) and CdCl₂:Cu²⁺ (R_eq=235 pm; R_ax=280 pm). Although an increase of Req decreases the energy of the first CT transition, CdCl₂:Cu²⁺ exhibits a value close to that of CuCl₄^2- due to the presence of axial ligands which rises the energy of the d-orbitals. The large Req distance in CdCl₂:Cu²⁺ is reflected however in the smaller separation between the two e_u(π+σ) → x²-y² and e_u(σ+ π) → x²-y² CT transitions. The present calculations indicate that the optical spectrum detected for (en H)₂ MnCl₄:Cu²⁺ displaying the first CT transition at 20800 cm^-1 is hard to be ascribed to an elongated CuCl₆^4- complex. By contrast it can reasonably be understood assuming a compressed situation.

Résumé

Les transitions de transfert de charge (CT) et de champ cristallin (CC) pour le complexe carré plan CuCl₄^2- et des complexes octaédriques CuCl₆^4- distordus sont évaluées en utilisant les états de transitions dans la méthode MS-Xα; la dépendance des transitions en fonction des distances metal-iigand est étudiée de manière à obtenir les structures électroniques des sels des cristaux où ces complexes interviennet. En particulier, le spectre optique de (enH)₂MnCl₄:Cu₂₊ est attribué à un complexe CuCl₆^4- distordu ou les deux Cl^- axiaux sont plus proches de Cu²⁺ que les quatre Cl^- équatoriaux.