We present a rigorous analysis of unique, wide electrochemical window solutions for rechargeable magnesium batteries, based on aromatic ligands containing organometallic complexes. These solutions are comprised of the transmetalation reaction products of PhxMgCl2-x and PhyAlCl3-y in different proportions, in THF. In principle, these reactions involve the exchange of ligands between the magnesium and the aluminum based compounds, forming ionic species and neutral molecules, such as Mg2Cl3+·6THF, MgCl2· 4THF, and PhyAlCl4-y- (y = 0-4). The identification of the equilibrium species in the solutions is carried out by a combination of Raman spectroscopy, multinuclear NMR, and single-crystal XRD analyses. The association of the spectroscopic results with explicit identifiable species is supported by spectral analyses of specially synthesized reference compounds and DFT quantum-mechanical calculations. The correlation between the identified solution equilibrium species and the electrochemical anodic stability window is investigated. This study advances both development of new nonaqueous solution chemistry and possible development of high-energy density rechargeable Mg batteries.