Chemistry of Polynuclear Metal Complexes with Bridging Carbene or Carbyne Ligands. Part 91. Steric Implications in the Reactions of the Complexes (M(CR)(CO)2(η-C5H5)) (M: Mo or W, R: C6H4Me-4, C6H4OMe-2, or C6H3Me2-2,6) with the Comp