Lithiation of M-C2H2 results significant enhancement of second-hyperpolarizability. The negatively polarized pi-surface of C2Li2 raises the molecular orbital energies which strongly favours both metal-HOMO to ligand-LUMO and ligand-HOMO to metal-LUMO charge transfer interactions. Due to the push electron effect by C2Li2 , the outermost 'ns' orbital of alkaline earth metal cannot induce charge transfer instead forms the excess diffuse electron density localized over the alkaline earth metal. Calculated vertical ionization energy of M-C2Li2 complexes are smaller than the corresponding isolated metals. CCSD calculated static and dynamic second-hyperpolarizabilities of M-C2Li2 complexes are many times larger than that of the corresponding M-C2H2 complexes.