Vibrational-Spectra of Salts of the Type-A^I₃[Mo₂X₉] with X = Cl, Br, I

Abstract

The far-infrared and Raman spectra of a series of alkali, ammonium and alkylammonium salts of Mo₂X₉^3- (X = Cl , Br, I) have been measured. All observed infrared-active vibrations, including lattice modes, together with certain Raman-active modes, have been assigned for the alkali salts of Mo₂Cl₉^3- on the basis of the D_6h unit cell symmetry which applies. The vibrational assignments for the corresponding bromide and iodide complexes follow in the same relative order as those of Mo₂Cl₉^3-. The splitting of degenerate modes for certain alkylammonium salts of Mo₂Cl₉^3- has been interpreted on the basis of C_2v symmetry applicable to these particular salts. By correlating the spectral changes observed with variation of the cation, it has been possible to distinguish the bridging-halogen deformation modes from neighbouring lattice modes.