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.