Infrared, Raman, resonance Raman, and excitation profile studies of Rh2(O2CCH3)4(PPh3)2 and its 18O and CD3 isotopomers

The resonance Raman spectra of RH2(O2CCH3)4(PPh3)2 and its 18O and CD3 isotopomers have been recorded at ca. 80 K. A band at 289 cm-1, denoted ν1, is assigned to ν(Rh-Rh) and is the dominant progression-forming mode. This band also forms combinations with bands attributable to the tetraacetate cage, such as ν(Rh-O) and δ(OCO), as well as with those of the axial PPh3 ligands. Excitation profiles of ν(Rh-Rh) and several other bands are shown to maximize under the 376-nm electronic absorption band, which, by way of Raman band depolarization ratio measurements, is assigned to an axial (z) polarized transition terminating in σ* (RhRh). Band assigned to ν(Rh-O) are shown to be dependent on both 18O and CD3 substitution. For Rh2(O2CCH3)4(PPh3)2. Raman spectra obtained with 514.5-nm excitation are also discussed. FTIR spectra (3500-40 cm-1) of Rh2(O2CCH3)4(PPh3)2 and its 18O and CD3 isotopomers are also presented and assigned. The results provide a firm basis for making vibrational band assignments for other dimetal tetracarboxylates. 18 references, 4 figures, 5 tables