The demonstration of single-site control over polymer stereochemistry by C2-symmetric ansatitanocene and zirconocene catalysts in 1984 and 1985 ushered in a new era in alkene polymerization catalysis.1,2 Since then, a plethora of ansa-metallocene ligand designs have been developed for the purpose of controlling the stereochemistry and molecular weight of the polymer product and for increasing the activity of the catalyst. Besides numerous ring substitution patterns as well as the replacement of one ring with another moiety (such as an amide ligand), different types of bridges have been used to alter both the geometry and electronic properties of the ansa-metallocene. For the most part, these bridges have consisted of relatively inert carbon-or silicon-based moieties. Apart from a few phosphorus-3,4 and arsenic5-bridged ansa-zirconocene complexes, examples of heteroatombridged group 4 ansa-metallocenes were scarce until about a decade ago, when the potential of the heteroatom to expand the functionality of the ansa-metallocene ligand and increase its influence on the polymerization properties of the catalyst was recognized. (Note: The term “heteroatom” is

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generally associated with any p-block element other than carbon; however, in this chapter it refers to any p-block element outside of group 14.)

So far, only boron, phosphorus, and arsenic have served as single heteroatom bridges in group 4 ansa-metallocene complexes, probably owing to the greater synthetic accessibility of these systems. ansa-Metallocenes with a single oxygen or nitrogen atom joining the π ligands have not been developed, although these atoms have been combined with one or two other heteroatoms6-9 or have been flanked by carbon and silicon to form the backbone of the bridge.10-15 This chapter will focus only on ansa-metallocenes containing heteroatoms within the backbone of the bridge. Heteroatombridged half sandwich complexes of the “constrained-geometry” type, in which theπ-ligand is linked to an amide ligand,16-18 are also outside the scope of this chapter.