Structure of highly concentrated aqueous electrolyte solutions of transition metal chlorides and bromides

The structures of the complexes dominating in highly concentrated and 1.0 mol dm^-3 aqueous solutions of first-row transition metal chlorides and bromides have been studied using large angle X-ray scattering (LAXS) and/or EXAFS. Of these systems, only zinc(II) chloride and bromide form inner-sphere complexes in dilute aqueous solutions. No inner-sphere complexes corresponding to dilute aqueous perchlorate solutions and [M(H₂O)₆](ClO₄)₂ solids were formed in 1.0 mol dm^-3 aqueous solutions of the cobalt(II), nickel(II) and copper(II) chloride and bromide systems, according to the superimposing EXAFS spectra of these samples. A cis-dibromo complex dominates in a highly concentrated aqueous manganese(II) bromide solution, as that found in the solid state. In highly concentrated solutions of cobalt(II) and nickel(II) chloride and bromide and copper(II) chloride, one inner-sphere complex is formed, while the remaining halide ions are in the second coordination shell. In an almost saturated copper(II) bromide solution (5.67 mol dm^-3), a polynuclear complex based on hydrated cis-CuBr₂ units is formed with a significantly different structure than that observed in the solid state, trans-[CuBr₂(H₂O)₂]. In a highly concentrated zinc chloride solution, 11.81 mol dm^-3, with less than two water molecules per ZnCl₂ formula unit, a [Zn₂Cl₄] complex is formed, where the zinc ions are three-coordinated in a triangular fashion. In a 5.60 mol dm^-3 zinc bromide solution, a dimeric [Zn₂Br₄(H₂O)₂] complex is formed, where the coordination around the zinc ions is distorted tetrahedral. These structures are different from those precipitated from saturated aqueous zinc chloride and bromide solutions, [Zn(H₂O)₆][ZnCl₄], [(H₂O)₅Zn-Cl-ZnCl₃] or [Cl₃ZnClZn(H₂O)₄ClZnCl]_n and [Zn(H₂O)₆][Zn₂Br₆]. An increased stability of high-symmetry complexes is observed in the solid state, while in aqueous solution, complexes with maximized hydration seem to be favored.