Density functional study of the tetraoxometallates of CrVI, MnVI and FeVI

Abstract

Local density approximation (LDA) density functional theory (DFT) calculations are reported for the tetrahedral anions [CrO₄]^2–, [MnO₄]^2– and [FeO₄]^2–. A range of STO basis sets is considered to determine the sensitivity of the optimised MO bond lengths and charge-density distributions. For a given complex, the computed bond lengths decrease smoothly as the basis set increases. The overall spread of predicted values is 0.04 Å, with experiment reproduced to within 0.02 Å for bases which include polarisation functions. The charge-density analysis across the series describes increasing covalence from [CrO₄]^2– to [FeO₄]^2–, while the computed relative bond strengths decrease. The latter correlate well with the observed increase in reactivity along this series. The computed values for the ligand-field parameter, Δ_tet, are about 2000 cm^–1 too high for d⁰[CrO₄]^2– and d¹[MnO₄]^2–, and 600 cm^–1 too low for [FeO₄]^2–. In conjunction with theoretical estimates of the Racah parameters from spin-unrestricted LDA calculations, the computed d–d spectrum of [FeO₄]^2– is in reasonable agreement with experiment.