Spin Hamiltonian for Cr III Complexes. Calculation from Crystal Field and Molecular Orbital Models and ESR Determination for Some Ethylenediammine Complexes

Abstract

The spin Hamiltonians of Cr(en)33+ and trans-CrCl2(en)2+ in single crystals of Co(en)3Cl3·3H2O, Co(en)3Cl3·NaCl·6H2O and trans-[CoCl2(en)2]Cl·HCl·2H2O have been determined from the ESR spectra. In the case of the trans-CrCl2(en)2+, ion it was found that the major axis for the spin—spin interaction lay along the C2 axis which is in the plane of the nitrogen atoms and rotates one ethylenediammine ring into the other. D and E for trans-CrCl2(en)2+ were calculated from both a crystal field model and from the results of an extended Hückel molecular orbital calculation. It was found that both calculations give comparable results and give the best results when the spin—orbit coupling parameter of the free ion is used. In the case of the MO calculation, the results agree with experiment when the N–Cr–N bond angle in the ethylenediammine ring is chosen to be 84°. For trigonal complexes of Cr III, D was also calculated using a crystal field model and again it was found that the best results were obtained when the spin—orbit constant was that of the free ion. These calculations also showed that large changes in D and E can be expected for crystal field symmetries that allow for configuration interaction between the ground state and an excited quartet state. The theory for g in these complexes was also developed. In the case of the near octahedral complexes it has been shown that g can be simply related to the charge on the Cr atom in the eg bonding orbitals.