Preparations and Infrared Spectra of some Metal Pentachlorobenzoates

The pentachlorobenzoates, NaOCOC₆Cl₅, 2H₂O, KOCOC₆Cl₅, H₂O, MOCOC₆Cl₅ (M = Cs or Tl), Hg(OCOC₆Cl₅)₂, RHgOCOC₆Cl₅ (R = Ph or p-MeOC₆H₄), and R₃SnOCOC₆Cl₅ (R = Ph or Me) have been prepared by neutralization, metathesis, or cleavage reactions. Vibrational assignments have been made for infrared absorption frequencies of, these compounds, pentachlorobenzoic acid, bispenta-chlorophenylmercury, and pentachlorophenylmercuric chloride. The carboxylate stretching frequencies indicate that Hg(OCOC₆Cl₅)₂, RHgOCOC₆Cl₅, and R₃SnOCOC₆Cl₅ (in chloroform) have unidentate pentachlorobenzoate ligands, whereas crystalline R₃SnOCOC₆Cl₅ compounds have bridging pentachlorobenzoate groups. Pentachlorobenzoic acid is dimeric in the solid state, and in toluene, carbon tetrachloride, and benzene there are both monomeric and dimeric species.