Spectroscopic, crystal, and halogen-exchange studies of halogenotrimolybdates(II)

Abstract

Quantitative solid (potassium halide disk) and hydrochloric acid solution adsorption spectra of halogenotrimolybdates(II) indicate that the compounds are of a common structural type, both in solid and solution, but that at least two kinds of chloro- or bromo-molybdate(II) are present. Variation of absorption intensities with hydrochloric acid molarity suggests the equilibrium (Mo₃Cl₁₂)^6-+Cl^- ↔ (Mo₃Cl₁₃)^7-, and moreover similar intensity changes occur when K₆Mo₃Cl₁₂ is in KCl. X-ray diffraction powder photographs of K₆Mo₃Cl₁₂ and A^I₇Mo₃Cl₁₃,H₂O (A^I = NH₄ or Rb) have been indexed on a pseudo-tetragonal unit cell and indicate 15 and 18 molybdenum atoms per unit oell respectively. The values are decisive for trimers (Mo₃) if it is assumed that all the compounds have similar structures. Halogen-exchange studies support the (Mo₃X₁₃)^7- parental structure, having four bridging, and nine terminal and labile, halogeno groups, since (NH₄)₇Mo₃Cl₁₃,H₂O rapidly exchanges only nine chlorine atoms in 1M hydrochloric acid.