ABSTRACT
The term metallocene defines a bis(cyclopentadienyl)metal complex; however, we extend it to cyclopentadienyl (Cp) complexes with main group elements (1). The cyclopentadienyl ligand (Cp) has played a major role in the development of organometallic chemistry since the structure of ferrocene (Cp2Fe) was identified in 1952 (2) and has continued to be the archetype of cyclic polyene ligands. Cyclopentadienyl is usually present as a pentahapto ligand in complexes with transition metal elements, where it exists formally as an anion (Cp ) acting as a six-electron donor. Over the last three decades, much effort in experiment and theory has been focused on transition metal metallocenes. In contrast, somewhat less is known about main group metallocenes due to the diversity of unusual structural properties and bonding phenomena between Cp and the main group elements (E). Recently, main group metallocenes have become important because of their structural fluxionality and synthetic utility in organometallic chemistry. However, most studies to date on main group metallocenes have concentrated on the structures of the neutral species in the gas phase or salt forms in the solid
state, and only a small number of investigations have focused on syntheses and reactivities (3). Moreover, only a limited number of theoretical studies have fo cused on main group metallocenes, because the different types of bonding charac ter between the main group element (E) and Cp ligand (due to less involvement of d orbitals compared to the transition metal metallocenes) result in numerous structural possibilities with different bonding patterns. In main group metallo cenes, 7r-type interactions become weaker due to the absence of d orbitals, and deviation from typical pentahapticity (T|5) is often observed. The broad range of electronegativities of main group elements (E) also leads to either ionic or cova lent bonding for Cp-E interactions. For example, bonding of Cp to s-block ele ments (Groups 1, 2) shows highly ionic character, while the /7-block elements (Groups 13, 14), which have electronegativities similar to that of transition met als, exhibit large covalent bonding character to the Cp ligand.
