Reactions of the phosphinidene cluster [Ru4(CO)13(µ3-PPh)] with tungsten acetylide complexes [WL(CO)3(CCPh)] (L = C5Me5 or C5H5) gave interconvertible isomers of formula [Ru4(CO)10(µ4-PPh)(CCPh){WL(CO)}]. The structures of the two C5Me5 derivatives were determined by X-ray diffraction, showing a novel WRu4P octahedral core arrangement, in which the acetylide ligand is co-ordinated to a WRu2 triangle with its C–C vector bridging the Ru–Ru edge. For one isomer the phosphinidene ligand is located at the position trans to the W atom, while in the second isomer it is at the cis position. Possible mechanisms for this rare example of skeletal isomerization are suggested.