Chemistry of Polynuclear Metal-complexes with Bridging Carbene Or Carbyne Ligands .91. Steric Implications in the Reactions of the Complexes [m(=cr)(co)2(eta-c5h5)] (m = Mo Or W, R = C6h4me-4, C6h4ome-2, Or C6h3me2-2,6) with the Compounds [mo(co)3(ncme)3] and [cu(thf)(eta-c5me5)] (thf = Tetrahydrofuran) - Crystal-structure of [mo3(mu-cc6h3me2-2,6)2(co)6-(eta-c5h5)2]o