d → d Spectrum and High-Spin/Low-Spin Competition in d~6 Octahedral Coordination Compounds: ab Initio Study of Potential Energy Curves

摘要

Ab initio calculations were performed, using the CASPT2 method and moderate-size basis sets, on several d~6 octahedral coordination compounds, Fe(CN)_(6~(4-)), Fe(NCH)_(6~(2+)), cis- and trans-Fe(CN)_2(NCH)_4 and Cr(CO)_6. The study concentrates on the six lowest states of the d → d spectrum (three singlets, one quintet, and two triplets states), and on the dependence of their energy on the metal-ligand equilibrium distance. It has been exended to a d~5 compound, Fe(CN)_(6~(3-)). The spin multiplicity of the ground states is correctly reproduced, and the metal-ligand distances and the vertical transitions energies, and dissociation energies are in good agreement with experiment. From the potential energy curves, it is possible to extract Racah's parameters, B and C, and the crystal field parameter Δ as a function of the metal-ligand distance. The dependence of Δ on this distance is rationalized in terms of ligand field theory. finally, the role of the triplet states in the spin transition is discussed.