The fortuitous synthesis in 19511 and structural elucidation of ferrocene in 19522 were monumental steps in what would become the field of organometallic homogeneous polymerization catalysis. Pioneering work in the realm of metallocene synthesis initially focused on the cyclopentadienide ligand, [C5H5]− (Figure 2.1), resulting in a family of structures having the general formula (C5H5)2MXn, where M is a transition, lanthanide, or actinide metal, and X is an anionic ligand (often Cl− or Br−) that completes the valence of the overall neutral metallocene.3 In 1953, the first metallocene

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[C5H5]− [C9H5]− [C13H9]−

FIGURE 2.1 Cyclopentadienide, indenide, and fluorenide are important ligands commonly found in stereoselective olefin polymerization catalysts.