Chromium carbonyl, Cr

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

G.A. Ozin, University of Toronto In our Cr/CO matrix cocondensation experiments (Angew. Chem., Int. Ed. Eng. 1975, 14, 292), we reported evidence for the facile formation of a binuclear chromium carbonyl complex Cr2(CO)i0 or Cr2 (CCOi x which could be described as square pyramidal Cr(CO)5 weakly interacting with either a Cr(CO)5 or Cr(CO)6 moiety in the vacant (sixth) site. As a result, the infrared spectrum of this "weakly-coupled" binuclear species closely resembled that of the mononuclear fragment Cr(CO)5. I would like to ask you, whether or not you have any evidence for the existence of such a binuclear species in your Cr(CO)6 /Xe cryogenic solutions following various photolysis treatments. 

This difference in behavior is felt to be due to the fact that Cr(co)e must first dissociatively lose a CO ligand, a fairly energetic process with an activation enthalpy of 40 kcal. (25.), prior to binding the formate ion. The once formed Cr(C0)502CH proceeds to Cr(C0)sH with subsequent production of H2 and C02 as described in the Scheme. Support for this interpretation was obtained by replacing Cr(C0)e with a chromium carbonyl species containing a labile ligand,... 

To test the validity of this mechanism, chromium carbonyl (1.0 g) was photolyzed under Ar at ambient temperature in a solution of methanol and hexamethylphosphoramide in the apparatus shown in Figure 5. The lamp was turned off periodically to check for the disappearance of slightly soluble Cr(C0) . Several photolyzing cycles were necessary to effect nearly complete conversion to the solvent-stabilized coordinately unsaturated species (equivalent to in Figure 4),... 

Italogenation catalyst. Chromium carbonyl catalyzes the monohalogenation of cyclohexane by CC14 (78% yield). Other cycloalkanes undergo the same reaction, liioniination can be effected in this way with CBrCl,. Other metal carbonyl complexes arc less active. Cr(CO)f, is actually more efficient than di-/-butyl peroxide. A free indical mechanism is involved. 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

CN depends not only on the composition of a coordination compound, but also on the type of a- and n-bonds present in it. Chromium carbonyl-pyridine complexes are the classic example to illustrate the difficult task of determining unambiguously the coordination numbers. Only a-bonds are present in Cr(py)(CO)5 and CN = 6 (12). However, in the a, n-complex 13, as well as in 9, the coordination number is quite questionable (compare formulae 9 IT). 

Moran et al. 41 attached the Cr(CO)3 moiety to tetrakis(phenylsilane) 32 (prepared by the hydrosilylation of tetraallylsilane with four equivalents of dimethylphenylsilane) by treatment with excess Cr(CO)6 in dibutyl ether—THF at 140 °C affording the air-stable, crystalline tetrakis(chromium carbonyl) dendrimer 33, which was also prepared by reaction of tetraallylsilane with [r76-C6H5Si(Me)2H]Cr(CO)3) (Scheme 8.9). 42a Reaction of the corresponding eight phenyl-terminated analogues afforded the partially metalated silane dendrimer 34 as the major product, even with an excess of Cr(CO)6. 

If the issue of n-complex formation with PMDA-ODA polyimide seems confused, the results of model compound studies have, in places, exacerbated the problem. Several research groups have turned to reactions of formally zerovalent chromium to model the chemistry of PMDA-ODA/Cr (58e, 60. 62) The studies assume that the chemistry of atomic Cr and of certain chromium carbonyl complexes is coextensive. Since atomic chromium has not been available as a benchtop reagent, popular sources of formally zerovalent chromium, disguised as Cr(CO)6 and (CH3CN)3Cr(CO)3, have been used. These compounds readily form (arene)Cr(CO)3 complexes (71). 

The diverse chemistry of chromium carbonyl complexes has its origin in the discovery of Cr(CO)6 in 1926 by Job and Cassal. Early developments in this area included the preparation of the (jj -arene)Cr(CO)3 family of organometallics by Nicholls and Whiting in 1959, the synthesis of the first structurally characterized carbene complex by Fischer and Maasbdl (1965), and a... 

The derivatives of chromium hexacarbonyl, Cr(CO)e (L)j , make up the single largest class of organochromium compounds, and a substantial number of these are Cr compounds as well. The most common synthetic route involves direct reaction of Cr(CO)6 and x L, but displacement of a weakly bound hgand from a carbonyl derivative is also frequently employed. Carbonyl dissociation is usually promoted with heating or UV irradiation. 

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