Synthesis of triangular mixed-metal clusters by the addition of copper electrophiles to an electron-rich rhodium-rhodium bond. X-ray crystal structures of [Rh2(.eta.-C5H5)2(.mu.-CO)(.mu.-Ph2PCH2PPh2)(.mu.-CuI)] and [Rh2(.eta.-C5H5)2(.mu.-CO)(.mu.-Ph2PCH2PPh2)(.mu.-AgO2CCH3)]