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(η-C5H5)2(μ-CO)(μ-Ph2PCH2PPh2)(μ-CuI)] and [Rh2(η-C5H5)2(μ-CO)(μ-Ph2PCH2PPh2)(μ-AgO2CCH3)]