Chemistry of di- and tri-metal complexes with bridging carbene or carbyne ligands. Part 45. Synthesis of the pentanuclear metal complexes M′M 2 Re 2 (µ-CC 6 H 4 Me-4) 2 (CO) 18 (M′= Ni or Pt, M = Cr or W) and related compounds; X-ray crystal structure of ReWPt(µ-CC 6 H 4 Me-4)(CO) 9 (PMe 3 ) 2