EXTENDED HÜCKEL MOLECULAR ORBITAL CALCULATION OF THE TEMPERATURE DEPENDENCE OF THE QUADRUPOLE SPLITTING OF [Fe(H2O)6] SiF6 AND KFeCl3

Abstract

A number of papers have appeared in which the temperature dependence of the quadrupole splitting has been treated, whereby covalency effects have been considered by introducing the orbital reduction factor k, with k ranging from 1.-0.7. There are, however, two problems : (1) k is unisotropic if the symmetry is lower than cubic, (2) the spin orbit coupling of the ligand electrons are usually neglected. These two problems have been treated here by using SCCEHMO [1] calculations on two examples, [Fe(H2O)6]SiF6 and KFeCl3, which have been previously delt with by other authors using ligand field theory [2, 3]. For both compounds the differences between the measured and the calculated quadrupole splitting were less than 0.1 mm/s within the whole temperature [3, 5] range. The most interesting result in case of KFeCl3 concerns the reduction of the spin-orbit coupling constant of the iron 3d electrons. The molecular coefficients of the 3d electrons have values of about 0.92 leading to a reduction of approximately 20 %. This effect is expected to increase with overlap between the central ion and the ligands and with increasing spin-orbit coupling constant of the ligand electrons. This result is in contrast with the relatively low value of the spin-orbit coupling constant of λ = 58 cm-1 found by V. Petrouleas et al. [3]. In this context the SCC-EHMO method proves to be superior to the ligand field model. The results obtained by Ingalls [2] on [Fe(H2O)6] SiF6 are nearly the same as in this work, because of the very low spin-orbit coupling constant of Oxygen (ζ2P = 150 cm-1). The nuclear quadrupole moment of the excited state of 57Fe tock on a value of 0.21 b in this work.