Computational Spectroscopy of the Cr-Cr Bond in Coordination Complexes

We report the accurate computational vibrational analysis of the Cr-Cr bond in dichromium complexes using second-order multireference complete active space methods (CASPT2), allowing direct comparison with experimental spectroscopic data both to facilitate interpreting the low-energy region of the spectra and to provide insights into the nature of the bonds themselves. Recent technological development by the authors has realized such computation for the first time. Accurate simulation of the vibrational structure of these compounds has been hampered by their notorious multiconfigurational electronic structure that yields bond distances that do not correlate with bond order. Some measured Cr-Cr vibrational stretching modes, ν(Cr₂), have suggested weaker bonding, even for so-called ultrashort Cr-Cr bonds, while others are in line with the bond distance. Here, we optimize geometries and compute ν(Cr₂) with CASPT2 for three well-characterized complexes, Cr₂(O₂CCH₃)₄(H₂O)₂, Cr₂(mhp)₄, and Cr₂(dmp)₄. We obtain CASPT2 harmonic ν(Cr₂) modes in good agreement with experiment at 282 cm^-1 for Cr₂(mhp)₄ and 353 cm^-1 for Cr₂(dmp)₄, compute ⁵⁰Cr and ⁵⁴Cr isotope shifts, and demonstrate that the use of the so-called IPEA shift leads to improved Cr-Cr distances. Additionally, normal mode sampling was used to estimate anharmonicity along ν(Cr₂), leading to an anharmonic mode of 272 cm^-1 for Cr₂(mhp)₄ and 333 cm^-1 for Cr₂(dmp)₄.