Metal carbonyls chromium hexacarbonyl

Another inorganic particle that has been prepared via RESS is iron oxide (Fe203) (75). The preparation involved rapid expansion of a Fe(N03)3 solution in supercritical water. The expansion was at 500°C and 100 MPa through 50-to 200-p,m-diameter orifices into an evacuated chamber. The Fe203 particles thus obtained were small and exhibited exceptional reactivities. In addition to inorganic oxides, several neutral metal carbonyls (chromium hexacarbonyl, dimanganese decacarbonyl, and triiron dodecacarbonyl) were processed via RESS to form micrometer-sized particles (76). The solubility of these compounds allowed the use of supercritical CO2 in the RESS processing. 

The binary metal carbonyls are named by giving the name of the metal followed by the name carbonyl, with the number of carbonyl groups indicated by the appropriate prefix. For example, Ni(CO)4 is nickel tetracarbonyl, whereas Cr(CO)6 is chromium hexacarbonyl. If more than one metal atom is present, the number is indicated by a prefix. Thus, Co2(CO)8 is dicobalt octacarbonyl, and Fe2(CO)9 has the name diiron nonacarbonyl. 

The mononuclear metal carbonyls contain only one metal atom, and they have comparatively simple structures. For example, nickel tetracarbonyl is tetrahedral. The pentacarbonyls of iron, ruthenium, and osmium are trigonal bipyramidal, whereas the hexacarbonyls of vanadium, chromium, molybdenum, and tungsten are octahedral. These structures are shown in Figure 21.1. 

Chromium hexacarbonyl was obtained by the checkers from Pressure Chemical Company, Pittsburgh, PA, and used without purification. It can be weighed 1n air as it is relatively non-volatile and air-stable. The usual precautions appropriate for a potentially toxic metal carbonyl should be employed, but the low volatility makes handling relatively easy. 

The reaction, of metal carbonyls with 1,3-diketones generally results in a complete displacement of carbon monoxide accompanied by oxidation of the metal to yield 1,3-diketonato complexes. For example, iron pentacarbonyl, chromium hexa-carbonyl, and molybdenum hexacarbonyl afford FefCgHjOOs,1 Cr(CsHr02)8,2 and Mo(CgH702)s,2,s respectively, when allowed to react with 2,4-pentanedione. 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

Enzymatically active NADH has been selectively produced by visible light photoreduction of NAD using [Ru(bipy)3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers and triethanolamine as electron donor (Wienkamp and Steckhan). There is continuing interest in the photogeneration of co-ordinatedly unsaturated species from metal carbonyls etc. which can act as or give rise to catalysts, e.g., for cis-trans isomerization and hydrogenation of alkenes. Ger-rity et al. have used chromium hexacarbonyl to make the first quantitative measurements of the distribution of atomic excited states produced by multiphoton dissociation of a metal carbonyl. The distribution of states turns out to be statistical rather than spin- or polarity-difierentiated. The use of perfluoromethylcyclohexane as solvent has enabled Simon and Peters to observe naked Cr(CO)s as a transient from Cr(CO)6. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

Other examples of transition metal carbonyls are nickel tetracarbonyl, Ni(CO)4, a colorless, toxic, flammable liquid that boils at 43°C and chromium hexacarbonyl, Cr(CO)6, a colorless crystal that sublimes readily. Ni(CO)4 is tetrahedral and Cr(CO)6 is octahedral. 

The catalyst system for the modem methyl acetate carbonylation process involves rhodium chloride trihydrate [13569-65-8]y methyl iodide [74-88-4], chromium metal powder, and an alumina support or a nickel carbonyl complex with triphenylphosphine, methyl iodide, and chromium hexacarbonyl (34). The use of nitrogen-heterocyclic complexes and rhodium chloride is disclosed in one European patent (35). In another, the alumina catalyst support is treated with an organosilicon compound having either a terminal organophosphine or similar ligands and rhodium or a similar noble metal (36). Such a catalyst enabled methyl acetate carbonylation at 200°C under about 20 MPa (2900 psi) carbon monoxide, with a space-time yield of 140 g anhydride per g rhodium per hour. Conversion was 42.8% with 97.5% selectivity. A homogeneous catalyst system for methyl acetate carbonylation has also been disclosed (37). A description of another synthesis is given where anhydride conversion is about 30%, with 95% selectivity. The reaction occurs at 445 K under 11 MPa partial pressure of carbon monoxide (37). A process based on a montmorillonite support with nickel chloride coordinated with imidazole has been developed (38). Other related processes for carbonylation to yield anhydride are also available (39,40). 

Conjugate reduction of enones. The alkali metal carbonylchromates reduce a, -unsaturated carbonyl compounds to the corresponding saturated carbonyl compounds in 4d-807o yield. They are comparable to potassium hydridotetra-carbonylferrate (6, 483-486), but are simpler to prepare because chromium hexacarbonyl is a stable solid and less toxic than iron pentacarbonyl. Examples ... 

Caution. Chromium, molybdenum and tungsten hexacarbonyls are volatile solids (Cr > Mo > W) and like all metal carbonyl compounds should be considered to be toxic. tert-Butylisocyanide has a pungent odor, and although many isocyanides are reported to exhibit no appreciable toxicity to mammals, it should still be handled with care. Carbon monoxide is evolved in these reactions and being an odorless, toxic gas, care should be exercised to carry out the reactions in an efficient ventilation hood with the apparatus venting into a well-ventilated region of the hood. 

Chromium carbenes can also be prepared by the so-called Semmelhack-Hegedus route. Chromium hexacarbonyl is first reduced to a nucleophilic pentacarbonyl dichromate dianion 23 with sodium naphthalenide or potassium carbide. Reaction of this dianion species with an acid chloride gives a metal alkoxide that can be quenched with an electrophile to provide the desired chromate ester 24. Alternatively, the dianion can be added to an amide carbonyl to give a tetrahedral intermediated which collapses to the chromate amide 25 on treatment with trimethylsilyl chloride. 

The electron configurations for the transition metals discussed here and in Appendix B are for individual metal atoms in the gas phase. Most chemists work with the transition metals either in the metallic state or as coordination compounds (see Chapter 25). A solid transition metal has a band structure of overlapping d and s orbital levels (see Section 13-7). When transition metal atoms have other types of atoms or molecules bonded to them, however, the electronic configuration usually becomes simpler in that the d orbitals fill first, followed by the next higher s orbital. This is illustrated by Cr, which has a 4s 3d electronic configuration as a free atom in the gas phase. But in the compound Cr(CO)5, chromium hexacarbonyl, which contains a central Cr atom surrounded by six neutral carbon monoxide (or carbonyl) groups, the chromium atom has a 3d electronic configuration. 

One frequently used solvent for the alkali metal reduction of metal carbonyls is liquid ammonia, in which alkali metals form blue solutions. These blue solutions are useful for some of the more drastic reductions, such as that of the very stable octahedral chromium hexacarbonyl to the anion [Cr(CO)s] ". Unfortunately, the reactivity of liquid ammonia toward most of the halide derivatives of interest for reaction with metal carbonyl anions has seriously limited its use in their preparation. Behrens and co-workers, however, have made an extensive study of the reactions of the [Cr(CO)5] anion prepared by the reduction of chromium hexacarbonyl with sodium metal in liquid ammonia these reactions will be discussed later. 

It is apparent that if the Lewis base is charged rather than neutral, the substituted metal carbonyl will have the same charge as the Lewis base. In this manner certain anionic metal carbonyls can be synthesized by the displacement of carbonyl groups with anionic Lewis bases. Frequently used for this type of reaction are the halide ions and cyanide ion. Many of these reactions have been carried out on the relatively stable hexacarbonyls of chromium, molybdenum, and tungsten, their derivatives or other related... 

Such aspects of metal carbonyl structure may be explained by consideration of the coordination number of the central metal atom as an important factor in determining the stability of metal carbonyls. As is the case with other transition metal derivatives such as the ammines, octahedral hexa-coordinate metal carbonyl derivatives seem to be especially favored. Thus, hexacoordinate chromium hexacarbonyl is obviously more stable and less reactive than pentacoordinate iron pentacarbonyl or tetracoordinate nickel tetracarbonyl. Moreover, hexacoordinate methylmanganese pentacarbonyl is indefinitely stable at room temperature (93) whereas pentacoordinate methylcobalt tetracarbonyl (55) rapidly decomposes at room temperature and heptacoordinate methylvanadium hexacarbonyl has never been reported, despite the availability of obvious starting materials for its preparation. 

The hexacarbonyls of chromium, molybdenum, and tungsten react with a variety of Lewis bases to form very stable also hexacoordinate substitution products. If the Lewis base is anionic a metal carbonyl anion will result as discussed above. 

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