Heterolytic Cleavage of Coordinated

One of the best examples of intermolecular heterolytic cleavage of /2-H2 is the protonation of ethers by extremely electrophilic cationic H2 complexes containing electron-withdrawing ligands such as CO56,51  

In addition to the above simple stoichiometric reactions, intermolecular heterolysis is observed or postulated in several types of exchange that can be catalytic, e.g., 

Similar Ru complexes with dppm ligands show equilibria between species observable 

In general, mechanisms of protonation of L include direct proton transfer via r-bond metathesis [Eq. (9.42)] and external base catalysis [Eq. (9.43)], where is a 

Equation (9.46) represents the first direct observation of equilibrium between an acidic H2 complex and a corresponding hydride complex with a protonated ancillary ligand.58 Ancillary ligands with no lone pairs, such as hydride, alkyl, and silyl groups, 

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A possible formulation for I is illustrated below. This could be formed by the heterolytic cleavage of a Ru-Ru bond an corresponding movement of a carbonyl from a terminal site to a bridging one to maintain the charge neutrality of both Ru atoms. The result would be to leave one ruthenium atom electron deficient (a 16 electron species) and capable of coordinating a two electron donor to give another intermediate I. ... 

Kinetic data were interpreted in terms of a mechanism in which initial heterolytic cleavage of H2 involved the basic 0 ligand coordination of olefin at the generated free equatorial site, followed by insertion to give alkyl and then protonation of the Pd-alkyl bond by the phenolic OH group, completes the catalytic cycle ... 

It may be reasonable to argue that this further activation is achieved in several ways. The acid-catalysis required for Gal and de Bruin complex [Rh(/c -bpa)(cod)](PF6) to react with dioxygen can be used to protonate the peroxo compoimd (Scheme 10) to a hydroperoxo species. This is a way to achieve further activation of dioxygen, since it decreases the nucleophilic character of the peroxo hgand and makes interaction with the coordinated olefin easier. Recent works by Moro-oka [88,89] and Braun [90] (Scheme 15) have shown that peroxorhodium complexes can be protonated to hydroperoxo compounds. However, the addition of a second mole of acid leads to hydrogen peroxide ehmination rather than to the highly electrophilic oxo species (M = O) that could result from the heterolytic cleavage of the O - O bond with removal of water. 

Chloro complexes of ruthenium(II) were found to hydrogenate maleic and fumaric adds to succinic add slowly at 60-80 °C and normal pressure of hydrogen. Non-activated alkenes lead to the production of ruthenium metal. The structures of the species involved are unknown. The mechanism involves coordination of the alkene followed by heterolytic cleavage of hydrogen, giving a ruthenium(II) hydride as the second step.41... 

Detailed studies with several Ru(H2) complexes showed that the yield of NH3 critically depended upon the pKa value of the Ru(H2) complexes (89). When the W-N2 complex was treated with 10 equiv of [RuCl(H2)(dppe)2]+ (dppe = l,2-bis(diphenylphosphino)ethane) with pi a = 6.0 under 1 atm of H2, NH3 was formed in up to 79% total yield (free NH3 plus NH3 released on base distillation). If the pKa of the Ru(H2) complex was increased to 10, the yield of ammonia decreased remarkably. Heterolytic cleavage of H2 was proposed to occur at the Ru center via nucleophilic attack of the coordinated N2 on the coordinated H2 where the coordinated N2 is protonated and a hydride remains at the Ru atom. Only a very limited number of reactions of bound N2 with H2 are known, e.g., Eq. (23) which slowly occurs in toluene over 1-2 weeks for a dinuclear Zr complex capped by macro-cyclic ligands with N and P donor atoms (90). 

This section is devoted to processes beginning with the heterolytic cleavage of a silicon bond by a neutral nucleophile. Thus, some aspects of intermediacy of cations of silicon having a coordination number of 4 are... 

The protonation of a thiolate donor, formation of a nonclassical r 2-H2 complex, release of H2 and addition of D2, and the heterolytic cleavage of this D2 by the concerted attack of the Lewis acidic metal center and the Brpnsted basic thiolate donor are essential steps. The acidic thiol deuteron can exchange with EtOH protons. The resulting free protons and the deuteride complex yield HD and the coordinatively unsaturated species that is the actual catalyst. The detailed mechanism comprises a considerably larger number of steps (and equilibria) (143). For example, the occurrence of r 2-D2 and [M(D)(SD)] intermediates that exchange with H+ should give rise to [M(D)(SH)]... 

Coordination chemistry has become a powerful tool for the control and the living nature of radical polymerization [79,80]. Various examples show that the role of initiator and counter radical can be played by organometallic species with an even number of electrons. Besides aluminum complexes used by Matyjaszewski, several other transition metals, metallocenes, and organolan-thanides with various ligands have been studied in controlled radical polymerization [79-97]. In some cases, a controlled polymerization was achieved [81,83-85,87,90-94,97]. However, the mechanism of the polymerization is not always known and it may happen that heterolytic cleavage of the active bond... 

Usually attack of the EPD occurs at the more positive, and attack of the EPA at the more negative part of the molecule. In both cases, coordination leads to an increase in polarity of the bond A +—B and finally to heterolysis with formation of stabilized cations or of stabilized anions 18, 19). It may be anticipated that a homolytic cleavage of A +—B is rather unlikely A" " acts as a much stronger EPA than A ", and B acts as a stronger EPD than B , so that maximum stabilization is usually achieved by heterolytic cleavage of the A +—B - bond. 

Jia and Morris reported that (dihydrogen)ruthenium(II) complex formulated as [RuCp(H2)L2] produces a proton by heterolytic cleavage of the coordinated H-H bond [77]. Of particular interest is that the pK values of these complexes closely depend on the ancillary ligand employed (pK = 4.9-9.0 in THE), suggesting that a decrease in the electron density of the metal increases the acidity of the dihydrogen complex. 

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