Molecular orbital calculations on transition metal complexes. Part VIII. Potential energy curves for some 3d6 complexes and their relation to Tanabe–Sugano diagrams

Abstract

Potential energy curves for the ground and some low energy excited states of a number of complexes with a 3d⁶ electronic configuration have been computed from INDO type MO calculations. The results agree extremely well with the known ground states of the complexes FeF₆^4–, CoF₆^3–, CO(H₂O)₆³⁺, and NiF₆^2–, in particular the crossover from high to low spin being predicted for the fluoro- and aqua-complexes of cobalt(III). In addition a successive decrease in metal–fluorine distance accompanied by a gradual increase in covalency for the series Fe^II→ Co^III→ Ni^IV is indicated in harmony with crystallographic and spectroscopic data. The calculated contraction in metal–ligand distance on passing from the high spin to the low spin configuration is ca. 0·06 Å for all complexes studied, in very good agreement with the value indicated by pressure dependent magnetic measurements. Differences in these potential energy curves give diagrams which essentially reproduce the main features of the crystal field Tanabe–Sugano diagrams.