d → d Spectrum and High-Spin/Low-Spin Competition in d6Octahedral Coordination Compounds: ab Initio Study of Potential Energy Curves

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.