Dinuclear complex formation in TaCl5-AlCl3 molten mixtures: Vibrational modes and thermodynamics

Raman spectroscopy has been used to obtain structural information on TaCl5-AlCl3 molten mixtures (XTaCl50=0-1) at temperatures up to 235 °C under static equilibrium conditions. The observed band wavenumbers and polarization characteristics along with the composition effects and the temperature-dependent features of the Raman spectra obtained for the molten mixtures indicate the occurrence of the dinuclear TaAlCl8 molecular liquid complex (m.p. ∼120 °C) in equilibrium with molten Ta2Cl10 and Al2Cl6. The spectra were interpreted in terms of a C2υ configuration for the TaAlCl8 molecule consisting of a TaCl6 octahedron sharing an edge with a AlCl4 tetrahedron. The spectral changes observed upon freezing indicate that the proposed configuration for the dinuclear complex is maintained in the polycrystalline state. The liquid TaAlCl8 complex undergoes a gradual dissociation to its components with increasing temperature. The thermodynamics and the extent of the reaction 1/2Ta2Cl10(l) + 1/2Al2Cl6(l) ⇆ TaAlCl8(l) were probed by applying a formalism for correlating the relative Raman band intensities with the stoichiometric coefficients, the equilibrium constant and the thermodynamics of the reaction. The enthalpy of the reaction was determined from the Raman data as ΔH° =  − 12.1 ± 1.5 kJ mol−1 and its value is discussed in terms of the coordination number and local geometry of Ta and Al atoms within the TaAlCl8 molecule.