Equilibrium constants carbonyl complexes

On the other hand, it may be expected that the metal ion forms a chelate complex with the substrate which involves the amino group as well as the carbonyl oxygen. In such a way, attack of OH- at the carbonyl carbon is facilitated even more by the electron-withdrawing effect of the metal ion. Furthermore, this assumption leads to a better understanding of the differences in the catalytic actions of the various metal ions, as far as these are not caused by differences in the equilibrium constants of complex formation. The mechanism of metal ion catalysis in these systems is similar to that of acid catalysis in so far as the metal ion is bonded to a basic site of the substrate to form an intermediate which readily undergoes nucleophilic attack at a neighboring position. 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

While the mechanism for the formation of the methoxy complex (14) is not established, it is significant that the dihydride Zr(C5Me5)2H2 is needed for the reduction of the CO coordinated in (13). A reasonable proposal for this reaction can be formulated if it is assumed that since complex (13) is formally d°, Zr—CO backbonding will not be of major importance, and that hydride complexes of the group 4 elements possess substantial hydridic character. The first assumption may lead to a more favorable equilibrium constant for carbonyl hydride formyl interconversion as in (5), while the second suggests H" attack in this sequence presumably on a coordinated formyl. If the latter results in Zr—H addition across C=0, then reductive elimination of a C—H bond leads to the observed product. This is shown in (21). 

Several directly measured values of AH° for homolytic dissociation of a metal-metal-bonded carbonyl in solution have been obtained (9). This was for the complexes [(n3-C3H5)Fe(CO)2 )2 where L = CO or a number of different P-donor ligands. The low value AH = 56.5 kJ mol-1 when L = CO was not unexpected for such a sterically crowded molecule. The P-donor substituents increased the steric crowding and displaced the equilibria in favor of the monomers but the effect seemed to be controlled more by AS° than AH°. In general metal-metal bond energies, however they may have been estimated, are too large to allow for direct measurement of equilibrium constants in solution in this way. 

Amine-metal-carbonyl reactions are limited to complexes in which the calculated C—O force constant (FC) is greater than ca. 17.2 mdyne A. Carbonyl complexes that have C—O force constants (FC) between 16.0 and 17.2 mdyne A react with amines to form an equilibrium mixture of reactants and products from which the product cannot be isolated e.g., in the reaction of [Mn(CO)3(mes- / )]PF6 (mes = mesitylene) and CyNHj in CHjClj the following equilibrium is established ... 

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