Density functional and Ab initio study of Cr(CO)_n (n = 1-6) complexes

Cr(CO)_n (n = 1-6) systems were studied for all possible spin states using density functional and high-level ab initio methods to provide a more complete theoretical understanding of the structure of species that may form during ligand dissociation of Cr(CO)₆. We carried out geometry optimizations for each system and obtained vibrational frequencies, sequential bond dissociation energies (BDE), and total CO binding energies. We also compared the performance of various DFT functionals. Generally, the ground states of Cr(CO)₆, Cr(CO)₅, and Cr(CO)₄, whose spin multiplicity is a singlet, are in good agreement with both previous theoretical results and currently available experimental data. Calculations on Cr(CO)₃, Cr(CO)₂, and CrCO provide new findings that the ground state of Cr(CO)₃ might be a quintet with C_2v symmetry instead of a singlet with C_3v symmetry, and the ground state of Cr(CO)₂ is not a linear quintet, as suggested by previous DFT calculations, but rather a linear septet. We also found that nonet states of Cr(CO)₂ and CrCO display partial C-O bond breakage.