METALLOCENES

Introduction

The cyclopentadienyl (Cp) ligand is a monoanionic ligand with the formula C5H5. The first characterized example of a cyclopentadienyl complex was ferrocene, Cp2Fe, which has an iron atom "sandwiched" between two planar Cp rings as shown on the left. For this reason, bis(cyclopentadienyl) complexes are sometimes called "sandwich compounds" ormetallocenes. Some metallocene derivatives, such as Cp2TiCl2, have their Cp rings tilted with respect to each other and are called "bent metallocenes", while complexes with only one Cp ligand have been colorfully described as having "half sandwich" and "3-legged piano stool" geometries. Bonding in Cp Complexes The normal bonding mode for Cp is 5 (pentahapto), for which several different resonance structures can be drawn for the bonding of an 5-Cp ligand to a transition metal complex. The one on the right makes it easy to remember that a Cp ligand donates either 5 or 6 electrons to a transition metal complex (depending on which electron counting formalism you use) as it looks like one alkyl ligand + two alkene ligands: While some of these forms are important in special cases (see ring slips below), a molecular orbital (MO) diagram best describes the bonding in a Cp complex. Consider the five MO's of a Cp ligand. If we have two of these, we can add and subtract various orbital combinations to generate the MO diagram for a metallocene. The lowest energy orbital, a1, does not have any favorable overlap with any of the metal d-orbitals. It has little interaction with the dz2 because the ligand p-orbitals lie on the dz2 conical nodal plane. The e1g set of degenerate orbitals overlaps quite well with the dxz and dyz orbitals on the metal, forming a strong set of pi-bonds. The e1u interaction between the metal px and py also gives some stabilization. Although the metal dx2-y2 and dxy can overlap with the e2g orbitals on the ligand, the degree of overlap is not very large and these levels are essentially non-bonding. The MO diagram for generic metallocenes, Cp2M is shown below. Notice that the Cp orbitals fill the six lowest orbitals. The next five unoccupied MO's shown in the box have little or no bonding character, which explains our observation above that metallocenes are known for a variety of d-electron counts. As one can infer from the MO description, most Cp complexes display completely delocalized bonding with equivalent C-C bond lengths in the C5 ring. The carbon-carbon bond distance of 143.3(6) pm in the Cp ring of ferrocene is somewhat longer than that observed in Na(TMEDA)Cp ( C-C distance of 138 pm) and other aromatic systems such as benzene (C-C distance = 135 pm). The longer C-C distance in ferrocene is the result of pi-backbonding from filled d-orbitals on Fe to the antibonding molecular orbitals on Cp. While the C5 ring of ferrocene is planar, the hydrogens are bent downwards towards the metal by about 5 degrees. This can be ascribed to a canting of the p-orbitals on carbon which permits better overlap of the ligand MO's with the metal orbitals (see the MO diagram below). The success of Cp ligands in contemporary organotransition metal chemistry can be traced to several key features: The M-Cp bond dissociation is large (ferrocene is stable to 400 degrees C). The ligand tends not to get involved chemically (although it certainly can from time to time). The ligand blocks several coordination sites. It has excellent NMR properties