The ruthenium cluster anion [HRu3 (CO) 8 (. mu.-PPh2) 2]-as a building block for the assembly of bimetallic tetranuclear clusters: synthesis, structure, fluxionality, and reactivity. Syntheses and structures of [H2Ru3Rh (. mu.-CO)(CO) 6 (PPh3) 2 (. mu.-PPh2)(PPhC6H4)] and [H2Ru3Ir (. mu.-CO)(CO) 7 (PPh3)(. mu.-PPh2)(PPhC6H4)]