The CASPT2 harmonic Cr-Cr stretching mode is at 282 cm -1 ; however, modes at 266, 267, 359, and 360 cm -1 consist of coupled metal-metal and metal-ligand stretching motions. We can compare ν(Cr 2 ) with experiment directly but note that the average of all modes involving significant Cr-Cr motion would give nearly the same value, 299 cm -1 , supporting Manning and Trogler's assignment of 340 cm -1 . Furthermore, these results reinforce previous conclusions based on the vibrational spectrum that the formally Cr 2 quadruple bond in this complex is much weaker, consistent with the electronic structure (vida supra). Although, to our knowledge, isotopic labeling experiments have not been performed for Cr 2 (mhp) 4 , ν(Cr 2 ) shifts to 287 cm -1 for Cr and 277 cm -1 for 54 Cr. Similar sized shifts are observed for the modes consisting of combinations of metalmetal and metal-ligand motions (Table ).