Additionally, ν(Cr 2 ) was computed for Cr 2 (dmp) 4 and the predominate Cr-Cr stretch is at 353 cm -1 . The mode is the largest of the three complexes, consistent with having the shortest bond distance and highest bond order, but remains significantly smaller than empirical estimates. The CASPT2 value for ν(Cr 2 ) is in good agreement with the estimate of this mode at 365 cm -1 based on averaging three bands observed in experiment. Specifically, three bands appear in experiments using the naturally occurring isotope (∼87% 52 Cr) at 345, 363, and 387 cm -1 (labeled as bands a, b, and c; Table ) and were assigned to involve Cr motions due to their shifts upon isotope substitution. Since several CASPT2 normal modes contributions from Cr-Cr, Cr-C and Cr-N stretching motions, further analysis including Cr and 54 Cr isotopes was performed (Table ).