Organometallic complexes with metal-metal bonds. Part 20. Synthesis of the first mixed-metal clusters with Ph2PCH2PPh2. Crystal structures of the metalloligated triangulo clusters: Pd2Co2(CO)7(Ph2PCH2PPh2)2 and Pt2Mo2(.eta.5-C5H5)2(CO)6(Ph2PCH2CH2PPh2)