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The cationic magnesium moiety of charged-separated magnesium organohaloaluminate complexes, relevant to rechargeable Mg battery electrolytes, typically takes the thermodynamically favourable dinuclear [Mg2Cl3]+ form in the solid-state. This thesis reports that judicious choice of Lewis donor allows the deliberate synthesis and isolation of the previously putative mononuclear [MgCl]+ and trinuclear [Mg3Cl5]+ modifications, hitherto only postulated as existing within electrolytic solutions, forming a comparable series with a common aluminate anion [(Dipp)(Me3Si)NAlCl3]-. Battery prototypes using these magnesium aluminates possessing different cationic aggregation as electrolytes were built in order to study the effect of the aggregation state on their performances. Despite early decomposition hindrance of some of the studied electrolytes, surprising behaviour was observed with the stability of the mononuclear species, which is highly enhanced compared to the higher aggregates. Rationally, this increased stability was attributed to the presence of the tetradentate Lewis donor, Me6TREN, used to control the aggregation of the mononuclear complex. Rechargeable batteries using magnesium organohaloaluminate electrolytes suffer from unwanted corrosion of the components of the battery cell (e.g., stainless steel parts), which is attributed to the presence of chloride ions. We devised a procedure to prepare sought-after magnesium compounds possessing the 'naked magnesium' dication [Mg*6THF]2+, by simply transmetallating the organic moieties from a magnesium reagent MgR2 to an aluminium reagent AlR3 (R = Ph or pyrrolyl). Irrespective of how successful the method is, these types of compound were revealed to be insoluble in ethereal solvents, presumably due to the strong opposite charge attraction emanating for the dication. By using a β-diketiminate ligand, the magnesium cation [(DippNacnac)Mg*2THF]+ was created, reducing the opposite charge attraction to give a soluble complex. These synthetic advances, alongside preliminary battery tests, give an insight into where the problems lie in the applicability of chloro-free compounds as electrolytes. Finally, as a potential alternative to aluminate complexes reported in the literature, the synthesis and characterization of analogous group 13 gallate complexes was successfully carried out.