METAL ISOTOPE SHIFTS IN THE RAMAN-SPECTRUM OF MO2(O2CCH3)4

Metal isotope substitution has been widely used for the assignment of metal-ligand vibrations of coordination compounds, but the application of this technique to compounds containing M-M bonds is very limited.2 In this work we report the results of a Raman study of Mo2(O2CCH3)4 in the normal isotopic (ca. 96Mo) and 92Mo forms. Mo2(O2CCH3)4 has a Mo-Mo bond distance of 2.09 A resulting from the Mo-Mo quadruple bond supported by four bridging acetate groups. Cotton and co-workers assigned the Raman feature at 404 cm1 as the symmetric Mo-Mo stretch, with no significant contribution from other stretching or bending modes, whereas Ketteringham et al. concluded that the Mo-Mo stretch contains about a 15% contribution from the O4M-MO4 deformation.34 Also, the question is open as to whether the contrast between the broad spread of Mo-Mo stretching frequencies and the narrow range of Mo-Mo bond distances reflects trends in bonding or simply the influence of coupling of the Mo-Mo stretch to Mo-ligand displacement coordinates.5. The purpose of this study is to (1) confirm the assignments of metal-dependent vibrations, particularly the metal-metal stretching mode, (2) infer the extent of mixing between the M-M coordinate and other internal displacement coordinates, and (3) provide data which will permit a more precise solution of the normal-coordinate vibrational problem. © 1979, American Chemical Society. All rights reserved.