Benzene-, chromium

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. 

Bis(benzene)chromium(0) is easily oxidized, and mixed complexes are obtained by means of substitution reactions. For example,... 

The reasoning applied above to 0 0 interactions can be extended to weak C-H 0 bonds. The presence of a short C-H 0 distance is not necessarily indicative of an overall attractive interaction, if the C-H donor and the O acceptor groups belong to anions, as illustrated in the following example. The structure of the mixed-valent species [(ri6-C6H6)2Cr] + [Cr03(0CH3)] [34] is based on columns of methoxychromate anions and of columns of bis-benzene chromium... 

Fig. 8 a The crystal is formed of columns of methoxychromate anions and of columns of bis-benzene chromium cations extending parallel to the c-axis. b The anions are apparently linked along the column via a short C-H O interaction (H O 2.381 A, C-H O 173°) between a methyl hydrogen and a chromate oxygen... 

Bis(benzene)chromium dichromate, 3851 Calcium chromate, 3926 Copper chromate oxide, 4223 Dibismuth dichromium nonaoxide, 0232 Lead chromate, 4243 Lithium chromate, 4236 Magnesium permanganate, 4691 Potassium dichromate, 4248 Potassium permanganate, 4647 Sodium dichromate, 4250 Sodium molybdate, 4713 Sodium permanganate, 4703 Zinc permanganate, 4710... 

X-ray dipole moments of formamide, of sulfamic acid, of benzene chromium tricarbonyl, and of water, obtained from /c-refinements, are in good agreement with those from other physical techniques. When hydrogen-atom positions are of crucial importance, as in the case of the water molecule, the availability of positional information from neutron diffraction becomes essential if accurate moments are to be obtained. In other cases, extension of the X-H bond to accepted values provides a reasonable alternative. 

The original concepts of metal-ligand bonding were essentially related to the dative covalent bond the development of organometallic chemistry has revealed a further way in which ligands can supply more than one electron pair to a central atom. This is exemplified by the classical cases of bis(benzene)chromium and bis(cyclopentadienyl)iron, trivial name ferrocene. These molecules are characterised by the bonding of a formally unsaturated system (in the organic chemistry sense, but expanded to include aromatic systems) to a central atom, usually a metal atom. 

In the bis(benzene)chromium molecule, each double bond of each benzene molecule may be considered as donating two electrons. The benzene molecules are... 

A second synthetic procedure for preparing silylated arene complexes, based on the metalation of a preformed rj6-arene complex, followed by treatment with an organochlorosilane, has been used only in one case so far namely, bis(benzene)chromium (47) ... 

Scheme 3.8 Metallation reactions of bis(benzene)chromium and bis(toluene)chromium with alkali metal alkyls and magnesium amides ...

The traditional names ferrocene, manganocene, ruthenocene, nickelocene, etc. are given to the respective bis(ri5-cyclopentadienyl)metal complexes. Ocene names should not be coined for isoelectronic species such as chromocene for bis(ri6-benzene)chromium, for structurally analogous... 

Ten grams of Bis (benzene) chromium (0) are added to a mixture of 200ml benzene and 100ml water contained in a 500ml auction flask which ia fitted with a stopper and an inlet tube that reaches almost to the bottom of the vessel. Air ia drawn through the flask until the complex has dissolved and the benzene layer has become practically colorless. The yellow-brown aqueous layer ia separated and filtered if necessary. It ia then treated with a aaturated solution of potassium iodide until no more precipitate forms. After cooling in ice the yellow product is filtered off, washed three times with 5ml of 95% alcohol, and finally with ether. The compound is dried in vacuo. 

Fluorine atoms in electron-deficient benzene derivatives, such as l-fluoro-2-nitroben-zene,149-150 l-fluoro-4-nitrobenzene.151 153 2-fluorobenzaldchyde,154"156 2-fluoroaceto-phenone,155156 4-fluoroacetophenonc,157 2-fluorobenzophenone.156 2- and 4-fluorobenzoni-trile,158 and 4-fluorophenyl trifluoromethyl sulfone,152,159 are easily substituted by N-nucleophiles. Even fluorine in electron-deficient sandwich complexes, such as tricarbonyl(t/6-fluoro-benzene)chromium (3), is substituted by amines.160161... 

Fischer and Hafner reported an alternate synthesis of bis-ir-benzene chromium using a reductive Friedel-Crafts method which probably also proceeds via a [Pg.231]

Bis(azidomethyl)oxetane, 1896 Bis(azidothiocarbonyl) disulfide, 1012a Bis(benzeneazo) oxide, 3484 Bis(jj-benzene)chromium(0), 3505 Bis(benzene)chromium dichromate, 3845 Bis(benzenediazo) sulfide, 3485 Bis(jj-benzene)iron(0), 3506 Bis(benzeneiron)—fulvalenediyl complex, 3828 Bis(jj-benzene)molybdenum(0), 3508... 

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