Carbon monoxide coordination

Insertion of CO is therefore always kinetically controlled. When an alkyl palladium species has formed, the open site will be occupied by a coordinating CO molecule. Carbon monoxide coordinates more strongly to palladium than ethene, even when the palladium centre is cationic. The reason for this is steric the cone angle of ethene is much larger than that of CO and the steric hindrance in the ethene complex is therefore much larger. If the barriers of activation for the insertion processes of ethene and CO are of the same order of... 

M(tj-CsH5)2 fragment with 7r-acceptor and -donor ligands and have emphasized the orbital interactions with conformational consequences, e.g., for carbene and olefin ligands. They have also discussed insertion reactions of olefins and carbon monoxide coordinated to this metal fragment 134). 

From the standpoint of the present review, we will focus on compounds in which unusual and potentially reactive features of CO coordination are revealed, and on certain classes of reactions in which the conversion of CO to other chemical entities is achieved. For the purposes of determining what a reactive mode of CO coordination is and in what way the CO ligand can be activated, it is first instructive to briefly review the primary modes of carbon monoxide coordination. These are illustrated as Lewis structures... 

The nature of bonding in the cationic metal carbonyls has been investigated by both vibrational and electronic spectroscopy, and molecular orbital calculations have been carried out these are consistent with a bonding scheme for carbon monoxide coordinated to a metal, consisting of a dative cr-bond from carbon to metal, augmented by a synergic metal-to-carbonyl dative 7r-bond (7, 57). 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

Figure 2.2 Schematic presentation of the carbon monoxide coordination at the transition metal...

The interaction of carbon monoxide with palladium salts produces under rela tively mild (but rigorously anhydrous) conditions such species as [Pd(CO)Cl2 ] 2, [Pdj CU(CO)j and probably, under higlier carbon monoxide pressure, species such as Pd(CO)2 CI3. Only under severe conditions are reduced species such as [Pd(CO)Xlrt X = Br. Cl formed, but since the catalytic reaction is carried out under a relatively mild carbon monoxide pressure, it is rather improbable that such reduced species are present in considerable amounts. For a general discus sion of the mechanism, however, the characteri .ation of the exact nature of the palladium(Il) carbonyl complexes formed m sint is irrelevant, ( nerally speaking, carbon monoxide coordination to a Pd(U) ion will produce a facile nucleophilic attack on CO by the alcohol. The question of whether such an attack is produced by free alcohol or by a palladium bonded alkoxy group, is worth examination. 

Carbon monoxide insertion into the copper-alkyl bond is indirectly shown by reaction of CO with dibutylcuprate(I), an anionic dialkyl derivative of dicoordinated copper(I). The product of the reaction, dibutylketone, may be rationalized by assuming carbon monoxide coordination to the anionic copper complex, followed by alkyl migration to the unstable anionic complex Cu[C(0)Bu](Bu) , with subsequent reductive elimination to the observed organic product. 

Although the reaction pathway is not known in detail, these results support an attack of an alcohol molecule on carbon monoxide coordinated to a cupric species in solution bearing chloride (and possibly hydroxo) ligands, for the generation of the alkoxy carbonyl intermediate (see equation 5). The hydroxo ligands buffer the system acidity. 

When Pd(II) is used as catalyst, the reaction can be initiated by an (alkoxycar-bonyl)paUadium species deriving from attack of the nucleophilic function of the substrate on carbon monoxide coordinated to palladium. This species then inserts the unsaturated... 

With an atomic number of 28 nickel has the electron conflguration [Ar]4s 3c (ten valence electrons) The 18 electron rule is satisfied by adding to these ten the eight elec Irons from four carbon monoxide ligands A useful point to remember about the 18 electron rule when we discuss some reactions of transition metal complexes is that if the number is less than 18 the metal is considered coordinatively unsaturated and can accept additional ligands... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

C=C stretch 80 C-13 chemical shifts 22, 53 C60 31,32 C60O isomers 54 carbon dioxide 120, 182 carbon monoxide 175,191 carbonyl series 84 carbonyl stretch 84, 220 in solution 244 Carmichael 136 Carpenter 152, 196 Cartesian coordinates 52, 286, 287 CASSCF keyword 228 CASSCF method 228,229,230,231, 232,233, 234,235 state-averaged 233... 

Reaction of diphenyl-2-thienylphosphine with Ru3(CO)l2 gives the tiVp) ti Ti C) coordinated species, 144, along with cluster 145 where two ligand molecules participate in coordination, one via the phosphorus atom and the C=C bond of the heteroring, and the other via the phosphorus atom only. P-Coordination in the products of such an interaction is known [95JOM(488)85]. Complex 144, the main product, interacts with carbon monoxide to yield the P-coordinated cluster, 146,... 

RuCl2(PPh3)2 reacts with 4-R2P-dibenzothiophene (R = Ph, p-Tol) and forms 303, in which the dibenzothiophene ligand is coordinated to ruthenium via the phosphorus and sulfur atoms [84JA5379, 87JOM(318)409]. The donor ability of the sulfur atom is relatively weak. Complex 303 (R = Ph) is able to add carbon monoxide and yield the monocarbonyl adduct. 

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

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