Cationic metal carbonyls protonation

The addition of a proton to a metal carbonyl compound may occur in either of two modes the formation of metal-hydrogen bond, or protonation of a ligand attached to the central metal atom. If the ligand protonated is an organic radical, a carbonium ion is produced, which may be stabilized by suitable delocalization of charge over the complex, including the central metal atom. Consequently, such protonated species may be legitimately considered as examples of cationic metal carbonyl compounds. 

Hydride abstraction of a hydrogen directly bonded to a metal atom has been used to synthesize cationic metal carbonyls. This may be accomplished by protonation, as outlined in Section C,l,f, or with a Lewis acid, such as boron trifluoride, in the presence of carbon monoxide (98). 

Interaction of the CO molecule with CuX-FER zeolites (X is an alkali-metal or proton as a co-cation) was investigated by IR spectroscopy and DFT calculations. An absorption band at 2138 cm 1 observed in IR spectra of CO on CuK- and CuCs-FER zeolites was assigned to a new type of CO adsorption complex on heterogeneous dual cation sites. CO molecule interacts simultaneously with Cu+ and alkali metal cations (via C- and O-end, respectively) in this type of complex. Interaction of CO with the secondary (alkali metal) cation led to a slight destabilization of the carbonyl complex. 

Because of the legion of unsaturated organic systems bonded to metal carbonyl residues, this method provides an abundance of cationic carbonyls. Our classification of the method employed is according to the nature of the protonation site. 

Many of the carbonyl cluster species are anions (cations being virtually unknown), hydrido species, or both. The relationships of these to each other and to neutral clusters in terms of electron count are the same as in simpler metal carbonyls, namely, one CO can be replaced by two hydrogen atoms, one H and one negative charge, or two negative charges. Protonation or deprotonation reactions are usually... 

Many additions to a,p-unsaturated carbonyl compounds, take advantage of coordination to the oxygen by a metal cation or a proton, or even just a hydrogen bond. This is true for hydrides and carbon nucleophiles. In such a situation, the LUMO coefficient is largest at the carbonyl carbon, but not at the p carbon. Thus, even soft nucleophiles can be expected to attack directly at the carbonyl carbon when Lewis or protic acid catalysis is involved. It is likely that the difference in the... 

No details are available on the evolution of the four-iron butterfly cation to methane, but further protonation of the framework and reductive elimination of CH4 seem likely. The four-metal butterfly framework appears to play a significant role in these reactions, particularly in activating carbon monoxide through II —CO formation. Significantly, the proton-induced reduction has been observed with other four- and six-metal carbonyl clusters, but the reaction does not appear to occur with clusters with fewer nuclei (248). By analogy with the findings in the iron system, this minimum metal nucleus number requirement suggests that n —CO may be involved in all of these reactions. 

Methods to generate the unsaturated intermediate that adds alkane include photolysis of metal-carbonyl or -dihydride complexes, thermolysis of alkyl hydride complexes, and abstraction of halide with a cationic reagent or protonation of an alkyl group with an acid containing a weakly coordinating anion. For example, the photolysis of Cp Ir(CO)j or Cp IrfPMejJHj generates Cp Ir(L) (L = CO or and photolysis of the... 

Oxidation of metal clusters may also be performed by reaction with Bronsted-acids through straightforward addition of protons to metal backbone. Thus, carbonyl clusters of ruthenium, osmium and iridium are stable in acids and may be pro toned without decomposition. The H-NMR spectra of these carbonyls in concentrated sulfuric or trifluoroacetic acid indicate the formation of cationic metal hydrides ... 

Many carbonyl and carbonyl metallate complexes of the second and third row, in low oxidation states, are basic in nature and, for this reason, adequate intermediates for the formation of metal— metal bonds of a donor-acceptor nature. Furthermore, the structural similarity and isolobal relationship between the proton and group 11 cations has lead to the synthesis of a high number of cluster complexes with silver—metal bonds.1534"1535 Thus, silver(I) binds to ruthenium,15 1556 osmium,1557-1560 rhodium,1561,1562 iron,1563-1572 cobalt,1573 chromium, molybdenum, or tungsten,1574-1576 rhe-nium, niobium or tantalum, or nickel. Some examples are shown in Figure 17. 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

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