Carbonyl, metal

The key difference between monomeric and dimeric metal carbonyls is that the latter compounds have a direct metal-metal bond joining the two 17-electron fragments. Two photoreactions that can occur with these dimers are the dissociation of a carbonyl ligand, or the homolysis of the metal-metal bond. Among the most studied dimers are the compounds M2(CO)io (M = Mn, Re). These compounds have the structure shown in Fig. 6.1 where there are no bridging carbonyl ligands between the metal centers. These cP-(f compounds show an intense absorption 

Strong evidence for a homolytic pathway comes from the photolysis of a mixture of Mn2(CO)io and Re2(CO)io in an inert alkane solvent, when the cross-product MnRe(CO)io is formed as an equilibrium product  

These photogenerated M(CO)5 radicals can also be spin trapped. Further support for the generation of such 17-electron radicals comes from the photoreaction of Mn2(CO)io with NO and CCU, when the products are Mn(CO)4NO and MnCl(CO)5, respectively  

These reactions involve the photogeneration of Mn(CO)5, which then combines either with the radical NO, or abstracts a chlorine atom from CCU, to give the respective Mn(I) compounds. If Re2(CO)io is photolyzed in the presence of water, the cubane-type compound [Re(/ -OH)(CO)3]4 is obtained. If M2(CO)io (M = Mn, Re) is photolyzed in the presence of a neutral donor ligand such as triphenyl-phosphine, the monosubstituted compound M2(CO)9PPh3 is formed  

We now move to the key 2e ligand types, CO, phosphines, and N-heterocyclic carbenes. We see how one ligand replaces another in a substitution reaction, Eq. 4.1, specifically for the classic case of the substitution of CO groups in metal carbonyls by phosphines, PRj. Hie principles involved will appear again later, for example, in catalysis. 

Whenever the donor atom of a ligand engages in a multiple bond, as in C=0, we have an unsaturated ligand. Along with PR3, these are 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

The frontier orbitals, and for M and C(lp) and CO(tv ) for CO, dominate the M-CO bonding. As shown in Fig. 4.1a and b, both C and O are sp hybridized in free CO. The singly occupied sp and orbitals on each atom form a a and a tv bond, respectively. This leaves carbon Py empty, and oxygen Py doubly occupied, and so the second tv bond is dative, formed by transfer of the 0 py) lone pair to the empty C py) orbital. This transfer leads to a C -0+ polarization of the molecule, which is almost exactly balanced by a partial C+-0 polarization of all three bonding orbitals because of the higher electronegativity of oxygen. The free CO molecule therefore has a triple bond and a net dipole moment very close to zero. 

This metal-induced polarization also activates the bound CO for chemical reactions, making the carbon subject to nucleophilic and the oxygen subject to electrophilic attack. The other ligands, L , modulate the polarization, as does the net charge on the complex. In L M(CO), 

The oxide C3O2 is well known (it is actually a derivative of the organic compound, malonic acid, HOOC—CH2—COOH) and an additional suboxide, C5O2, has been reported. The structure of the first suboxide is known to be I and that of the second is presumably II. 

No compounds C2O2 or C4O2 have been described it has been suggested that whereas suboxides with an odd number of carbons may be stabilized by forms such as III, such structures may not be drawn for suboxides with even numbers of carbon atoms (Exercise 2). 

Carbon monoxide has been found to be surprisingly reactive toward the metals in Group VIII, in both their oxidized and unoxidized states. A sizable number of compounds exist in which one or more CO molecules are attached to a metal atom through the carbon typical of these are nickel tetracarbonyl, Ni(CO)4, iron pentacarbonyl, Fe(CO) cobalt carbonyl hydride, Co(CO)4H platinum carbonyl chloride, Pt(CO)2Cl2 and more complicated molecules such as Co4(CO)i2. 

Of these reactions in which CO coordinates with the oxidized form of metals, the most important is the reaction with the iron atom in the hemoglobin of the blood, leading to carbon monoxide poisoning. This iron atom is part of a ring structure similar to that in chlorophyll (Chap. 6), and it ordinarily takes part in the metabolic process by reversibly forming a Fe—0—0— compound with the oxygen of the air. If carbon monoxide 

The most familiar cyanides, those of the alkali metals, may be made in quantity by high-temperature reactions involving alkali metal amides (for example, NaNHo) or carbonates  

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CO. Alkynes will react with carbon monoxide in the presence of a metal carbonyl (e.g. Ni(CO)4) and water to give prop>enoic acids (R-CH = CH-C02H), with alcohols (R OH) to give propenoic esters, RCH CHC02R and with amines (R NH2) to give propenoic amides RCHrCHCONHR. Using alternative catalysts, e.g. Fe(CO)5, alkynes and carbon monoxide will produce cyclopentadienones or hydroquinols. A commercially important variation of this reaction is hydroformyiation (the 0x0 reaction ). 

Nickel tetracarbonyl Ni(CO)4 was the first metal carbonyl to be discovered, by Mond in 1890 it is obtained by passage of carbon monoxide over nickel metal heated to 320 K. It is a volatile, toxic liquid (b.p. 315 K), and has a tetrahedral structure. It has considerable stability, but inflames in air it is believed that in the structure... 

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

Metalation Metalations Metalaxyl [137414-52-9] Metal borides Metal carbonyls... 

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Conjugated dienes, upon complexation with metal carbonyl complexes, are activated for Friedel-Crafts acylation reaction at the akyhc position. Such reactions are increasingly being used in the stereoselective synthesis of acylated dienes. Friedel-Crafts acetylation of... 

E. Caldeiazzo, R. Eicoli, G. Natta, I. Wendei, and P. Pino, eds.. Organic Synthesis via Metal Carbonyls, Vol. 1, Wdey-Inteiscience, New York, 1968. 

In atomization, a stream of molten metal is stmck with air or water jets. The particles formed are collected, sieved, and aimealed. This is the most common commercial method in use for all powders. Reduction of iron oxides or other compounds in soHd or gaseous media gives sponge iron or hydrogen-reduced mill scale. Decomposition of Hquid or gaseous metal carbonyls (qv) (iron or nickel) yields a fine powder (see Nickel and nickel alloys). Electrolytic deposition from molten salts or solutions either gives powder direcdy, or an adherent mass that has to be mechanically comminuted. 

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Bimetallic Complexes. There are two types of bimetaUic organometaUic thorium complexes those with, and those without, metal—metal interactions. Examples of species containing metal—metal bonds are complexes with Ee or Ru carbonyl fragments. Cp ThX(CpRu(CO)2), where X = Cl or 1, and Cp7Th(CpM(CO)2), where M = Ee or Ru, have both been prepared by interaction of CP2TI1X2 or Cp ThCl [62156-90-5] respectively, with the anionic metal carbonyl fragment. These complexes contain very polar metal—metal bonds that can be cleaved by alcohols. 

Fig. 13. The stmctures of closo metallaboranes where O represents BH , CH (a) [< /(9j 0-( q -C H )Ni(B22H22)] (b) closo-l]l-[v[-Q ]) -l]l-53i] pri Closo metallaboranes can also be formed by the direct interaction of polyborane and metal carbonyl clusters. For example.

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

The double-bond length in 1,3-butadiene is 0.134 nm, and the ingle-bond, 0.148 nm. Since normal carbon—carbon single bonds are 0.154 nm, this indicates the extent of double-bond character in the middle single-bond. Upon complexing with metal carbonyl moieties like Fe(CO)2, the two terminal bonds lengthen to 0.141 nm, and the middle bond shortens even more to 0.145 nm (18). 

The hydroformylation reaction is carried out in the Hquid phase using a metal carbonyl catalyst such as HCo(CO)4 (36), HCo(CO)2[P( -C4H2)] (37), or HRh(CO)2[P(CgH3)2]2 (38,39). The phosphine-substituted rhodium compound is the catalyst of choice for new commercial plants that can operate at 353—383 K and 0.7—2 MPa (7—20 atm) (39). The differences among the catalysts are found in their intrinsic activity, their selectivity to straight-chain product, their abiHty to isomerize the olefin feedstock and hydrogenate the product aldehyde to alcohol, and the ease with which they are separated from the reaction medium (36). 

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