Metal arene complexes metallocenes

Table 2. Vertical ionization potentials [IP] and oxidation potentials ( °ox) of metallocenes and (arene) metal sandwich complexes.

As far as organometallics are concerned, and important exception is represented by the (per)alkylated metallocenes and the related bis(arene) complexes of various transition metals. These compounds have been intensively studied in connection with the magnetic properties of their CT complexes, mainly by Miller and co-workers [11]. These studies led to the discovery of the first organometallic compound displaying bulk ferromagnetic properties (vide infra). 

Several CT complexes of metal bis(arene) compounds containing, among others, the familiar organic acceptor TCNQ have been reported [71]. None of these materials has been shown to display physical properties superior to those of the metallocene systems. One very important exception is related to the reaction of bis(benzene) vanadium (which is isoelectronic with [MnCpf ]) with TCNE, which affords a material with unprecedented properties [72]. The amorphous material obtained displays bulk ferromagnetism at room temperature and its even exceeds the decomposition temperature of the sample of about 350 K. The compound is no longer an arene complex as it has an empirical composition corresponding to V(TCNE)2 1/2(CH2C12). 

Thermochemical data for arene complexes are less abundant than for metallocenes. This is due in part to incomplete combustion of the compounds. The energies of dissociation of metal-arene bonds were obtained from mass spectra for most of the complexes. However, data obtained by this method are somewhat higher (by ca lOOkJmol" and even higher). In some cases, even the reverse order of the relative stabilities for a given series of compounds is obtained. Table 10.8 gives energies of dissociation of metal-arene bonds which are obtained by calorimetric methods. 

Neutral bisarene metal complexes are moderately soluble in organic solvents. They are thermally stable, often up to 570 K. They sublime in vacuum at ca 370 K. Most bisarene neutral compounds may be easily oxidized to mono and dipositive cations which, as expected, are considerably more stable. In weakly basic solutions, the cation [Cr(PhH)2] may exist in air without decomposition for several weeks. Compared to metallocene sandwich compounds, arene complexes have considerably weaker basic properties. 

Bis-arene metal sandwich complexes are known for all the transition metals, and their structure and chemistry resembles that of metallocenes. However, the even ligands are less strongly bonded to metals than the odd Cp ligand, and their chemistry suffers from easy decomplexation and is less rich than that of metallocenes. 

Arene cyclopentadienylmetal and arene metal carbonyl complexes are two other classes of materials that are metallocene-like in structure. The first report of the synthesis of tiie T) -mesitylene-Ti -cyclopentadienyliron cation in 1957 by Coffield et al. was a seminal event in this research field. However, it was the 1985 report on the polymerization of T -dichlorobenzene-Ti -cyclopentadienylruthenimn with diphe-nohc compounds, forming compounds such as 10, by Segal that caused research in this area to expand. 

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