Morris N. Levy

Proximity Effect in Arene Pi-Complex Formation

A proximity effect in arene π-complex formation from the reaction system of an aryl Grignard reagent and anhydrous chromic chloride was investigated. The yields of arene π-complexes and any accompanying reaction products were compared by the systematic variation of the aryl Grignard reagent employed. Phenyl Grignard gave the best yield of π-complex followed by the benzyl, mesityl and β-ethyl phenyl analogues. The styryl and phenylacetylenyl analogues did not form any π-complexes. The coupling reaction of benzyl groups gave a new π-complex, π-bibenzyl-π-toluene chromium, along with toluene, bibenzyl and trans-stilbene. Trans-stilbene was found without forming a trace amount of cis-stilbene. A mechanism for this stereospecific and α-hydrogen elimination coupling reaction is proposed.

Nature of the π-Complex Bond

The bonding in transition metal complexes has been described by three different theories: crystal field theory (CFT), valence bond theory (VBT), and molecular orbital theory (MOT). Detailed descriptions of these three approaches are given in the standard inorganic texts and are not repeated here. However, some general statements concerning the applicability of these various bonding descriptions for metal π-complexes are noted.

Structure and Structure Determination

The area of the structural elucidation of compounds has made giant strides since a few decades ago when the chemist was generally restricted to the use of chemical reactions, such as ozonolysis, hydrogenation, and melting points, in establishing the nature of newly synthesized materials. Although some may lament the current trend away from pure chemistry and toward physical methods, one cannot help but be impressed by the array of tools available to the modern chemist and the vast amount of detailed structural information resulting from their knowledgeable application.

Spectroscopic and Magnetic Properties of Metal π-Complexes

Selected physical properties such as spectroscopy and magnetic chemistry reveal useful data on the general skeletal arrangement, bond strength, energy, and valency of metal π-complexes. In this chapter some of the details of infrared spectroscopy (IR), nuclear magnetic resonance (NMR), mass spectra, Mossbauer spectroscopy, magnetic susceptibility, and oxidation state are discussed in terms of the characterizations of metal π-complexes.

History, Classification, and Nomenclature

Metal π-complexes possess a relatively new type of direct carbon-to-metal bonding that cannot be designated as one of the classic ionic, σ-, or π-bonds. It is now known that a large number of both molecules and ions such as monoand diolefins, polyenes, arenes, cyclopentadienyl ions, tropylium ions, and π-allylicions can form metal π-complexes with transition metal atoms or ions.

Reactions of Metal π-Complexes

Metal π-complexes are susceptible to a wide range of chemical reagents. However, the three major groups of metal π-complexes—π-olefin, π-cyclopentadienyl, and π-arene metal—demonstrate distinctly characteristic reactions. π-Cyclopentadienyl complexes (metallocenes) exhibit a high degree of aromaticity and undergo many typical aromatic substitution reactions. The π-arene complexes, on the other hand, do not exhibit a discernible degree of aromaticity. This comparative behavior invites diverse speculation on the exact nature of the π-complex bond. The reactions of most π-olefin complexes often parallel those of uncomplexed olefins. Differing behavior is generally explained on the basis of the strength and stability of the metal-olefin bond, which resists attack.

Catalysis Involving Metal π-Complex Intermediates

In recent years it has become evident that many metal-catalyzed reactions proceed via a substrate metal π-complex intermediate. Commercially, the most significant of these include the polymerization of ethylene, the hydroformylation of olefins yielding aldehydes (oxo process), and the air oxidation of ethylene-producing acetaldehyde (Wacker process).

Preparation of Metal π-Complexes

A large number of metal π-complexes have been prepared since initial work began, and rapid development in this field continues. Among the methods of preparation most frequently used are substitution, elimination, cyclization, ligand or metal exchange, σ-π rearrangements, and redistribution reactions. Examples of these methods are illustrated in this chapter. The equations presented are not necessarily complete, since the emphasis is placed not on a balanced equation but rather on the particular reaction product desired.