Raman spectra of multiply bonded metal species

Abstract

The Raman and i.r. spectra of a series of dimeric molybdenum and rhodium compounds are reported. The metal–metal stretching frequencies are found to fall in the region 400 ± 4 cm^–1 for the molybdenum carboxylates Mo₂-(O₂CR)₄(R = Me, CF₃, Et, Prⁿ, Ph, and C₆H₁₁) and 342 ± 8 cm^–1 for the (MO₂X₂)^4– containing species (X = Cl and Br) indicating a difference in metal–metal bond strength between the two series of compounds which has not previously been recognised from X-ray data. The metal–metal stretching frequency of the dimolybdenum tetrakis-(perfluoroacetate) is lowered by the attachment of donor ligands along the metal–metal axis. The metal–metal stretching frequencies for the rhodium carboxylates Rh₂(0₂CR)₄ and Rh₂(O₂CR)₄,2L (R = Me and Et, L = MeOH, H₂O, Me₂SO, and Ph₃P) fall in the region 288–351 cm^–1 and decrease with increasing donor strength of the attached ligands.