Ligand stabilizing

Stabilization of /j -alkyl and -aryl derivatives of transition metals can be enhanced by the judicious inclusion of various other stabilizing ligands in the complex, even though such ligands are known not to be an essential prerequisite. Particularly efficacious are potential IT acceptors (see below) such as AsPh3, PPh3,... 

It appeared to be a logical consequence to transfer this synthetic principle to more suitable metals like ruthenium and introduce bulky, kinetically stabilizing ligands at the metal. An interesting example for this approach is the complex 78. The latter is synthesized from Cp RuCl(PR3)2 with ClMgCH2SiMe2H through 77 by a thermal Si — H activation reaction. 

In experiments where Mono Lake water was acidified to remove carbonate and bicarbonate ions and again adjusted to pH 10, more than 90 percent of the soluble plutonium moved to the sediment phase. When carbonate ion concentration was restored, the plutonium returned to solution—strong evidence of the importance of inorganic carbon to solubility in that system(13). Early studies with Lake Michigan water, which has low DOC, had also implicated bicarbonate and carbonate as stabilizing ligands for plutonium at pH 8(14). This latter research characterized the soluble species as mainly anionic in character. 

Dehydrohalogenation of (Ar)MCl2(Mes PH2) has also been explored in the absence of a stabilizing ligand and shown to result in the formation of 66 in the case of the Ru complex, presumable via the RU2P2 dimer 65 [104]. The formation of 68 was shown to result when the dehydrohalogenation was executed in the presence of 2-butyne, which has metallocycle 67 as likely intermediate [104]. 

Nearly all the presently known compounds contain one or more w-acceptor ligands (e.g., CO, Cp, RjP) on the transition metal. These ligands may, as has classically been assumed for alkyls (72), dissipate some of the negative charge density on the central metal. However, it will be stressed later (Section IV) that such stabilizing ligands are unnecessary, and their role may in any case be more complex. 

It has been speculated in the past that it might be possible to isolate the first Au(I) fluoride LAuF [182], if disproportionation into metallic gold and Au( 111) can be avoided by stabilizing ligands L, such as (PR3)3AuF [264]. This has just been achieved. Laitar et al. [185] were able to isolate a compound with an N-heterocyclic carbene ligand... 

For the first time it was shown that SET is dependent on the nature of the stabilizing ligands as well as on the composition of the solution, surrounding the functional array. 

The reactions of a neutral 10 as well as a cationic dihydrido(acetato)osmium complex 12 with acetylenic compounds were examined (Scheme 6-17) [11-13]. A vinyU-dene 99, an osmacyclopropene 100, or a carbyne complex 101 were obtained, depending on the starting hydrido(acetato) complexes or the kind of acetylene used. In any case, the reaction proceeded by insertion of a C C triple bond into one of the two Os-H bonds, but the acetato ligands do not take part in the reaction and act as stabilizing ligands. 

Relatively few investigations involving palladium carbonyl clusters have been carried out, partly because palladium per se does not form stable, discrete homometallic carbonyl clusters at room temperature in either solid or solution states.114,917-922 Nevertheless, solution-phase palladium carbonyl complexes have been synthesized with other stabilizing ligands (e.g., phosphines),105,923 and carbon monoxide readily absorbs on palladium surfaces.924 Moreover, gas-phase [Pd3(CO)n]-anions (n = 1-6) have been generated and their binding energies determined via the collision-induced dissociation method.925... 

It can thus be concluded that the formation of the 1 1 adduct is favored by higher temperature and the absence of stabilizing ligands such as phosphine, although the structure of the silanes seems to be the most important factor. 

In order to explain the competitive formation of the 1 1 and 1 2 adducts and the formation of the 2,6-octadienyl rather than the 1,6-oc-tadienyl chain, a mechanism was proposed (62, 69) in which the insertion of one mole of butadiene to the Pd—H bond gives the 77-methallyl complex (68) at first, from which 1-silylated 2-butene is formed. At moderate temperature and in the presence of a stabilizing ligand, further insertion of another molecule of butadiene takes place to give C5-substituted n-allyl complex 69. The reductive elimination of this complex gives the 1 2 adduct having 2,6-octadienyl chain. In the usual telomerization of the nucleophiles, the reaction of butadiene is not stepwise and the bis-n--allylic complex 20 is formed, from which the l, 6-octadienyl chain is liberated. 

The feasibility of the above model rests on the formation of Cu(I) in the copper(II)-ascorbic acid system. A recent study firmly established that Cu(I) can indeed accumulate in the presence of a stabilizing ligand, Cl-, and in the absence of 02 (14). The actual form of the rate law is determined by the relative rates of Cu(I) formation and consumption, and further studies should clarify how the stability and reactivity of copper(I) are affected by the presence of various components and the conditions applied. 

Rhodium(II) acetate complexes of formula [Rh2(OAc)4] have been used as hydrogenation catalysts [20, 21]. The reaction seems to proceed only at one of the rhodium atoms of the dimeric species [20]. Protonated solutions of the dimeric acetate complex in the presence of stabilizing ligands have been reported as effective catalysts for the reduction of alkenes and alkynes [21]. 

Various methods have been used to convert precatalysts into the active species [7]. Ethylene can be easily displaced from the central atom of the corresponding complexes in solution, even at room temperature. CO-ligands in carbonyl complexes can conveniently be removed photochemically [8], Increasing the temperature is a further common method used to labilize precatalysts with respect to stabilizing ligands [9],... 

Transition metal hydrides play a key role in the catalytic homogeneous isomerization of olefins. The pure hydrides such as HCo(CO)4 can function as the catalyst, or transition metals complexed to stabilizing ligands can function as catalysts the catalysis almost certainly proceeds through hydride intermediates in many cases. 

The CPop intermediate is the j5-cuprio ketone intermediate widely debated in mechanistic discussions of conjugate addition (cf. Scheme 10.3). On the basis of recent theoretical analysis, two limiting structures for CPop may now be considered these are shown in the bottom box in Scheme 10.5. The reason for the exceptional stability of CPop as a trialkylcopper(III) species can be readily understood in terms of the j5-cuprio(III) enolate structure, with the internal enolate anion acting as a strong stabilizing ligand for the Cu state [82]. 

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