Chromium carbonyl complexes benzene

Carbazole will react with 1 or 2 mol of ferrocene in hot decalin in the presence of aluminium-aluminium chloride producing crystalline derivatives in which either one or both" of the benzene rings is linked to iron, 25 and 26, respectively. The sandwich compound 25 was deprotonated to 27 with sodamide in liquid ammonia. A chromium carbonyl complex 28... 

The normal behaviour of pyridines in the presence of metal cations is complexation involving donation of the nitrogen lone pair to the metal centre. This means that for simple pyridines, formation of 7r-complexes like benzene-chromium carbonyl complexes, does not take place. However, if the nitrogen lone pair is hindered, then ry -complexes can be formed. 

In chromium carbonyl complexes of indole, the metal is associated with the benzene ring, hence nucleophilic additions take place in that ring, usually at C-4 the example shows the relatively unusual attack at C-7 this regioselectivity can be induced to revert to the usual C-4 if an indole with a bulky A-protecting group is utilised. ... 

Commencing with alkyl group rotations, there have been two studies of the stereodynamics of chromium carbonyl complexes with 77 -hexaalkylbenzene. For the complex [ Cr(CO)2L 2(/i-N2)] (L = ry -hexaethylbenzene), NMR band-shape changes were attributed to slowed ethyl group rotation with a barrier, AG (300 K), of 46.0 3.0 kJ moP. The ligand hexa-n-propyl-benzene adopts a D3d symmetry with the alkyl substituent alternately up and down with respect to the observer. This geometry is retained in the chromium tricarbonyl complex and a decoalescence phenomenon observed in its NMR spectrum was... 

Consequently, these charge effects are reflected in the carbonyl stretching frequencies (87, 88). It has recently been found from studies of the far infrared spectra that the metal-carbon stretching frequencies also support the theory (89). These charge-distribution effects are supported further by the observed dipole moments (90-92). Thus the dipole moments of the chromium tricarbonyl complexes of hexamethylbenzene, benzene, and methylbenzoate lie in the order 6.22, 4.92, and 4.47 /x, respectively. The relationship of charge effects to chemical reactivity is described below. 

Detailed vibrational assignments have been carried out on arene chromium and arene molybdenum tricarbonyl complexes (18, 19). Splitting of the E band in the carbonyl region was observed for substituted benzene chromium tricarbonyl complexes but not in (CgHg)Cr(CO)3 itself, showing that the concept of local symmetry (Cg ) is of very restricted validity when discussing the C—O stretching vibrations in such complexes (18). 

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. 

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... 

Following this initial discovery, several additional routes to benzene-chromium tricarbonyl and related complexes were developed, the most convenient of which involves the simple refluxing of the metal carbonyl and arene with concurrent loss of carbon monoxide 96, 97,184, 187). 

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