Reaction of triangulo-clusters [Pt3(m-CO)3(PR3)3] with hexafluorobutyne. The X-ray crystal structures of [Pt2(CO)2(PR3)2(m-h2:h2-CF3C.tplbond.CCF3)] (PR3 = PPh3 or PCy3) and [Pt2(CO)2(PBzPh2)(m-h1:h1-CF3C.tplbond.CCF3)2]