d→d spectrum and high-spin/low-spin competition in d6 octahedral 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 Delta as a function of the metal-ligand distance. The dependence of Delta on this distance is rationalized in terms of ligand field theory. Finally, the role of the triplet states in the spin transition is discussed.