Bis arene complexes

The commonest six-electron ligands are benzene and substituted benzenes. Ligands of this type are termed arenes . They form complexes with most of the transition elements, although the variety of derivatives is somewhat smaller than for the cyclopentadienyls. Bis(benzene)complexes are known for M 

Bis(benzene)chromium has the 18-electron configuration and is thus iso-electronic with ferrocene. It forms brown-black crystals, m.p. 281°C and is thermally rather stable, although it is readily oxidized in air. In the crystal the molecule has a sandwich structure in which the rings are eclipsed symmetry) 

The first product in these reductions is the tetrachloroaluminate of the complex cation  

For chromium this can be reduced by alkaline aqueous dithionite or, with loss of half the product, allowed to disproportionate in aqueous solution. 

The green complex bis(benzene)molybdenum can be prepared similarly in up to 70% yields 

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Bis(arene)chromium complexes applications, 5, 345 in engineered materials, 5, 344 Bis(arene) complexes... 

From solutions of GaCl2 in hot 1,3,5-trimethylbenzene the colorless crystalline bis(arene) complex [C6H3(CH3)3]2Ga+ GaCl4 (XLVI) has been isolated (158). The structural study reveals a bent sandwich structure for the cationic moiety, as indicated schematically in Fig. 9. The tetrachlorogallate tetrahedra are once more only weakly coordinated, this time via one chlorine atom each to the Ga(I) centers of two bis(arene)gallium units, forming a chainlike coordination polymer. The steric requirement of the mesityl ligands is... 

Sixteen-electron bis(arene) complexes of formally zero-valent zirconium, hafnium, and titanium have been made by cocondensation of the metal atoms with bulky ligands such as 1,3,5-tri-t-butylbenzene71 ... 

Bis(arene) complexes of ruthenium (99), first prepared in relatively low yield by the Fischer-Hafiier synthesis (see Fischer-Hafner Synthesis) (MCb/AlCb/arene) are now readily available through solvento cations [(arene)RuS] + (S = acetone, nitromethane), which in turn are made from the dimeric half-sandwich halides [(arene)RuCl2]2 (Section 6.3) with Ag+ with the appropriate arene as solvent (Scheme 23). This method allows a whole range of functionalities to be incorporated to at least one of the arenes. The dications (99) + are of relatively high solvolytic stability and are decomposed by DMSO only to form [(arene)Ru(DMSO)3] +. 

There are just few examples of authentic lanthanide complexes in the oxidation state zero. Bis(arene) complexes of the lanthanides (l,3,5- Bu3C6H3)2Ln (Ln = Sc, Y, La, Nd, Pr, Sm, Gd, Tb, Dy, Ho, Er, Lu) have been synthesized by cocondensation of metal vapors (see Metal Vapor Synthesis of Transition Metal Compounds) with 1,3,5-tri(ferf-butyl)benzene at 75 K. A sandwich structure with coplanar arene ligands has been proven by X-ray crystal structure analysis of the Gd and Ho complexes (Figure 86a). 

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). 

Equation (4)).121 This molecule has also been prepared in solution by reaction of the phosphaalkyne with the bis(arene) complex, (r/ -(, I IsMc)2Ti, or by salt metathesis of TiCl with (KP3C2tBu2) (n = 2, 3) in refluxing toluene. The sandwich complex undergoes a [2 + 2]-cycloaddition with Bu C=P to yield [Ti(r/i-P3C2tBu2)(r/,r/2 P403 Bu3) 202, which has also been characterized by X-ray diffraction. 

The most general way to prepare bis( -arene) complexes uses the arene, a metal halide, an electron-pair acceptor acid such as AIX3, and Al metal as a reducing agent ... 

To our knowledge, no recent papers have appeared on the reactivity of rare-earfh bis arene complexes. 

Finally, rare-earth bis arene complexes have been shown to polymerise ethylene with narrow polydispersity and high turnover numbers (Cloke, 1993). 

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