Significance: Closed-shell metal–metal bonds are experimentally and theoretically substantiated as bona fide polar-covalent/ligand-unassisted/ground-state Cu(I)–Au(I) d ¹⁰ –d ¹⁰ bonds. Counterintuitively, replacing some of the gold content with copper leads to stronger metal–metal bonding (despite the lesser relativistic effects in Cu vs. Au) and higher phosphorescence quantum efficiency [despite much lower spin-orbit coupling constants ( ξ _SOC ) for 3d vs. 5d orbitals]. The former results are attributed to greater orbital mixing between (n+1)s/p orbitals of one metal with filled nd ¹⁰ orbitals of the other metal in the heterometallic vs. homometallic system, whereas the near-unity photoluminescence quantum yields and higher extinction coefficients in mixed-metal (Au/Cu) heterometallic vs. homometallic systems are attributed to symmetry reduction. The latter results are promising for enabling societally useful technologies (e.g., Ir-/rare-earth-free LEDs).