Gaseous complexes — Their role in chemical transport

The literature about gaseous complexes consisting of two different metals and one halide is reviewed. With respect to chemical transport the most important properties of gaseous complexes are their appreciable thermodynamic stability and their volatility. This is clearly illustrated in the case of RbSnCl₃, whose vapour in equilibrium with the melt at 980 °K contains around 25 mole% of RbSnCl₃ (g) and whose vapour pressure is around 20 times higher than that of RbCl. Gaseous complexes of the type Mⁿ⁺L₂Cl_n+6 (M = Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Nd³⁺, Cr³⁺; L = Fe³⁺, Al³⁺) can, in an atmosphere of L₂Cl₆(g), have vapour pressures up to 10⁷ times higher than the vapour pressure of MCl_n. The importance of such gaseous complexes for the chemical transport of MCl_n and of compounds containing Mⁿ⁺ will be discussed.