Arenes transition metals

By coordinating to arenes, transition metals can facilitate ring lithiation by decreasing the electron density in the ring and acidifying the ring protons. We shall consider briefly the two most important metal-arene complexes in this regard—arenechromium tricarbonyls and ferrocenes. 

The first arene-transition metal complexes were prepared in the 1950s1314 and it was immediately recognized that the added polarizibility or electron deficiency would promote addition of nucleophiles to the arene ligand. A number of cyclohexadienyl complexes were characterized following nucleophilic addition, but the question of inducing the ipso hydrogen to depart was not answered (equation 1). 

Arene Transition Metal Chemistry, 13, 47 Arsonium Ylides, 20, 115... 

Apart from bis(benzene)chromium and other bis(arene) metal compounds, a huge number of mono(arene) transition metal complexes were also prepared in the golden 1950s . In 1957, Fischer and Ofele reported the synthesis of (C6H6)Cr(CO)3, the first representative of the family of arene metal carbonyls [64], The original preparative procedure (a sealed-tube reaction of Cr(CO)6 and Cr(C6H6)2 at 220°C) was soon replaced by a more convenient synthesis, based... 

The first reported arene-transition metal trifluorophosphine complex [Cr(i/6-C6H6)(PF3)3] was obtained by UV irradiation of [Cr(i/6-C6H6)(C0)3] with PF3 (method A), whereas the corresponding thermal reaction afforded instead the mixed carbonyl trifluorophos-... 

The 7t-toluene complex is unique because (1) it is the first example of a -arene bound to a diorganoNi(II) species, and (2) the t-arene is more labile than in any other 7t-arene transition-metal complex. Exchange with other arenes is rapid even below RT . Because Because of this high lability, ( j -toluene)Ni(CgF5)2 can serve as a homgeneous catalyst for hydrogenation and polymerization, and where the arene can be replaced by a host of ligands". 

In arene-transition metal complexes the ligand is considered to occupy three coordination sites. With this view it is interesting to compare the results of arene substitution with the above data for cycloheptatriene. The... 

That the kinetically derived relative adsorption constants, Kab, decrease with the numbers of alkyl substituents is surprising because alkyl substituents increase the basicity of the benzene ring and stabilize Tl -arene transition metal complexes. The directly measured adsorption coefficients of benzene, toluene, p-xylene and mesitylene on a cobalt catalyst at 89 °C do increase with the number of methyl groups and the rates of hydrogenation decrease in that order. A consensus regarding the significance of the kinetically determined adsorption constants has not been reached. ... 

II. Some General Aspects of Arene Transition Metal Chemistry... 

SOME GENERAL ASPECTS OF ARENE TRANSITION METAL CHEMISTRY... 

Arene Transition Metal Chemistry TABLE V—Continued... 

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