This invention relates to a Salen type ligand including three or more quaternary ammonium salts of nitrate anions, to a trivalent metal complex compound prepared from this ligand and a method of preparing the same, to a method of preparing polycarbonate by copolymerizing an epoxide compound and carbon dioxide using the complex compound as a catalyst, and to a method of separating and collecting the catalyst from the copolymer after copolymerization. This catalyst used to copolymerize an epoxide compound and carbon dioxide can be more simply prepared, and has lower catalyst preparation and recovery costs, and higher activity, compared to conventional catalysts.