Bonding in metallocenes

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

The bonding in metallocenes has been the subject of numerous theoretical and spectroscopic papers (3, 4, 25, 79, 87, 112, 181, 182, 186, 201) and will be discussed only briefly here. A typical simplified... 

Three topological models for covalent bonding in metallocenes have been... 

Three topological models for covalent bonding in metallocenes have been defined[108] (Fig. 14.2 (b,c) also shown in Figure 14.2(a) is the purely electrostatic model) ... 

Coordination creates additional problems also. Consider the metal-Cp bond in a metallocene. One option is to have five bonds from the metal to each carbon. A second option is to have a single bond connecting to a dummy atom at the center of the Cp ring. 

Many of the properties of the group 15 element diheteroferrocences are very similar to ferrocenes and other metallocenes. It seems justified to regard the diheteroferrocenes as perturbed ferrocenes just as we regard the group 15 heterobenzenes as perturbed benzenes. Thus, it is very clear that the elements phosphorus, arsenic, antimony, and bismuth can take part in 7r-bonding in a manner similar to carbon. 

Its aromaticity cannot, of course, be tested by attempted electrophilic substitution, for attack by X would merely lead to direct combination with the anion. True aromatic character (e.g. a Friedel-Crafts reaction) is, however, demonstrable in the remarkable series of extremely stable, neutral compounds obtainable from (15), and called metallocenes, e.g. ferrocene (16), in which the metal is held by n bonds in a kind of molecular sandwich between the two cyclopentadienyl structures ... 

On a more qualitative level, the bonding in the more stable isomer lb can be explained on the basis of the general molecular orbital scheme for bent (C2v) metallocenes containing 14 valence electrons, as shown in Fig. 5. The localization of three electron pairs in bonding orbitals (lal, 2 i, 2b2) is primarily responsible for the Si-Cp interaction the absence of a silicon orbital of a2 symmetry imposes the presence of a ligand-based non-bonding orbital. Structural adjustment from D5d (ferrocene type) to C2v... 

To check for the flatness of the potential energy surface relative to the swing motion of the Zr-C bond in the equatorial belt of the metallocene, we performed a geometry optimization at the B3LYP level, with the angle 0 fixed at 61.7°, which is the value it assumes at the MP2 level. The optimized structure resulted to be only 0.8 kcal/mol higher in energy relative to the... 

The double-bond composition varies in a complex manner with changes in metallocene and monomer concentrations because the orders of dependence of the various P-hydride transfer reactions on monomer and metallocene are not the same [Liu et al., 2001c Zhou et al., 2001]. Vinylidene content decreases with increasing monomer concentration, but increases with increasing metallocene. The trends for vinylene content are the opposite, while trisubstituted double-bond content is relatively unaffected by monomer and metallocene concentrations. 

Numerous spectral determinations have been made on this class of compounds. In some cases, very detailed spectra have been obtained on both vapor and solutions down to 180 mp. In a few of the simple cases attempts have been made to assign the bands although, since a universally accepted picture of the bonding in the metallocenes has yet to arrive, the assignments perhaps should be regarded as preliminary. 

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