Physical Organic Chemistry of Transition Metal Carbene Complexes. 8.1Kinetic and Thermodynamic Acidities of Alkoxyalkylcarbene Pentacarbonyl Complexes of Cr, Mo, and W in Aqueous Acetonitrile. Dependence on Metal, Alkyl Group, and Alkoxy Group

Fischer Carbene Complexes: Beautiful Playgrounds To Study Single Electron Transfer (SET) Reactions

The knowledge of the reactivity of Fischer carbene complexes in electron transfer processes is still in the early stage of development, but interesting advances are foreseeable in this young branch of metal-carbene chemistry. Although these compounds have a dual reactivity (which makes them good substrates for oxidation and reduction processes), their behavior towards chemical electron transfer (ET) reagents was unknown until very recently. This article covers the progress accomplished in the reactivity of these compounds towards chemical ET reagents (C(8)K or SmI(2)), as well as the use of nonconventional sources of electrons, such as electrospray ionization (ESI) to induce ET processes. Special emphasis will be made on the effect of the structure of the starting carbene in the outcome of the reaction and in discussing the different mechanisms proposed.

Physical organic chemistry of transition metal carbene complexes. 25. Kinetic and thermodynamic acidities of substituted (methylthiophenylcarbene)pentacarbonyl tungsten(0) and (Benzoxymethylcarbene)pentacarbonyl tungsten(0) in aqueous acetonitrile. Evidence for transition state imbalances

A kinetic study of the reversible deprotonation of substituted (methylthiophenylcarbene)pentacarbonyltungsten(0) ((CO)(5)W=C(SC(6)H(4)Z)CH(3)) and of substituted (benzoxymethylcarbene)pentacarbonyltungsten(0) ((CO)(5)W=C(OCH(2)C(6)H(4)Z)CH(3)) by primary aliphatic and secondary alicyclic amines in 50% MeCN-50% water (v/v) at 25 degrees C is reported. From the dependence of the deprotonation rate constants on amine basicity and on carbene complex acidity (variation of Z), Brønsted beta(B) and alpha(CH) values, respectively, were obtained. The alpha(CH) values were found to be smaller than the beta(B) values. These results indicate a transition state imbalance in which the loss of the carbene complex stabilizing pi-donor effect of the OCH(2)Ar and SAr groups lags behind the proton transfer. These findings confirm a previously formulated hypothesis as to how pi-donor groups attached to the carbene carbon of carbene complexes can affect transition state imbalances and mask the experimental manifestation of such imbalances. It is also shown that the transition state structure of the reactions examined in this work is subject to changes with changing amine basicity and carbene complex acidity; these changes can be expressed by p(xy)() cross correlation coefficients, which are positive.

Physical organic chemistry of transition metal carbene complexes. 23. Kinetic and thermodynamic acidities of cationic benzothienyl- and selenylcarbene complexes of rhenium in aqueous acetonitrile

The pK(a) values of a cationic selenyl- (5H(+)) and a benzothienylcarbene complex (6H(+)) and rate constants for the reversible deprotonation of these complexes by water, carboxylate ions, primary aliphatic amines, secondary alicyclic amines (5H(+) only), and OH(-) (5H(+) only) were determined in 50% MeCN-50% water (v/v) at 25 degrees C. In comparison with neutral Fischer-type carbene complexes such as 1H, the cationic complexes 5H(+) and 6H(+) are much more acidic, and the intrinsic barriers to proton transfer are substantially higher. This paper discusses a variety of factors that contribute to these differences, with the most important ones being that 5H(+) and 6H(+) are cationic, which makes the C(5)H(5)(NO)(PPh(3))Re moiety a stronger pi-acceptor than the (CO)(5)M moieties, coupled with the fact that the deprotonated forms of 5H(+) and 6H(+ )are aromatic molecules.