Chemistry of polynuclear metal complexes with bridging carbene or carbyne ligands. Part 63. Synthesis of eight-membered-ring metallacycles: X-ray crystal structures of [Pt4W4(-CR)(3-CR)3(-CO)(CO)7(?-C5H5)4]CH2Cl2, [Ni2Pt2W4(-CR)(3-CR)3(-CO)(CO)7(?-C5H5)4]CH2Cl2(R = C6H4Me-4), and [Ni2Pt2W4(3-CPh)4(C