Preparation of tetranuclear heterometallic clusters by condensation of tungsten acetylides [W(CO)3(CCR)(-C5H5)] with acetylide clusters [WRu2(CO)8(CCR)L](L =-C5H5 or -C5Me5, R = Ph or C6H4F-p). Crystal structures of [W2Ru2(CO)9{CC(C6H4F-p)}(-C5H5)2] and [W2Ru2(CO)6(CCPh)2(-C5H5)(-C5Me5)]