The 62-electron cluster [Ru4(CO)13(μ3-PPh)] (1) reacts with phosphalkynes RCP [(R = tBu, Ar‘ (2,4,6-tBu3C6H2)] to yield a series of unsymmetrically capped bisphosphinidene clusters characterized by a ketene substituted μ4-PR ligand. The reaction pathway was followed by 31P-NMR spectroscopy and reveals that, in the first step of the reaction, a CO substitution of 1 occurs in order to form an intermediate with a σ- or π-bonded phosphaalkyne. After scavenging of CO the chain compound [Ru4(CO)12(μ3-PPh)(μ3-PC(CO)R)] [R = tBu (2), Ar‘ (7)] is created. Further CO elimination leads to the closo-complexes [Ru4(CO)10(μ-CO)(μ4-PPh)(μ4-PC(CO)R)] [R = tBu (3), Ar‘ (4)]. Due to steric reasons the nido-cluster [Ru3(CO)9(μ3-PPh){μ3-PC(CO)tBu}] (5) is the final product in the reaction starting from Ar‘CP. The molecular structures of the complexes 2−5 are discussed.