First principles account for large changes in electronic structure and bonding from LaCu to LaCuMg and LaCuMg4

•Increasing amounts of Mg in LaCu bring drastic electronic and mechanical changes.•From LaCu to LaCuMg and LaCuMg4 cohesive energy compressibility increase.•Mg substructures keep the metallic behavior with the least charge transfer in LaCuMg4.•Electrons brought by Mg are bonding states concomitantly with enhanced cohesive energy.