DENSITY-FUNCTIONAL STUDY OF THE TETRAOXOMETALLATES OF CRVI, MNVI AND FEVI

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 M=O 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 angstrom, with experiment reproduced to within 0.02 angstrom 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, DELTA(tet), are about 2000 cm-1 too high for do [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.