Lability toward decomposition

Organometallic compounds with a 17-electron configuration are often labile toward associative ligand exchange. Radical chain mechanisms are well established for phosphine substitution on metal carbonyl hydrides (Scheme 23), the 17-electron chain carrier being in most cases non hydridic. This mechanism, however, was also shown to operate for OsH2(CO)4 via the 17-electron hydride complex OsH(CO)4 [137]. Thus, phosphine addition to the radical prevails over the dimerization, which indeed occurs in the absence of phosphine [33] (section 6.5.7), and over other possible decomposition pathways. The second step of the chain propagation process in Scheme 23, for this osmium system, is another example of atom transfer to a hydride radical (section 6.5.6). 

In general, ESI can be well applied to ionic metal complexes and related compounds if these are soluble to at least 10" M in solvents suitable for the method [21-23]. Whether conventional or nanoESI should be employed basically depends on the tendency of the respective compounds towards decomposition. Labile complexes or compounds that are strongly adhesive to surfaces are preferably ana-... 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The Lherapeulic utility ol an enzyme preparation is largely dependent on iis stability as finally formulated. A number of chemical modifications have been employed and include binding to inert surfaces and encapsulation lo Increase the resistance of ihese intrinsically labile macrnmolecules to decomposition. Unfortunately, some of these modifications can interfere with the optimal kinelic performance of the enzyme. The development and deployment of recombinant DNA technologies have made significant contributions toward meeting the goal of muss production of human enzymes for human use. 

Also remarkable in Scheme 7.2 is the appearance of a final decomposition product associated with a new EPR signal, indicative of the presence of the so-called g = 2.03 dinitrosyls, which are biologically relevant and labile species, active toward vasodilation.57 Their general formula is [Fe(L)2(NO)2], in a pseudotetrahedral arrangement, with L = thiolates, imidazolates, and so on as coligands. In the reported reaction conditions of Scheme 7.2, L should be necessarily cyanide. 

This reaction is remarkably clean considering the lability of the intermediates and the complexity of its mechanism, which requires that the initial lithiation and trapping with TIPSCl at the 5-position be faster than other processes, including the second lithiation at the 4-position. However, it suffers from the lack of reactivity of the 2-lithiothiazole. At -78°C, this material is unreactive towards hindered lactones and primary alkyl halides, though it will react with more reactive alkyl halides and aldehydes, and at temperatures above about -70 °C, it is unstable and undergoes decomposition at a rate competitive with alkylation. A survey of various electrophiles is shown in Table 1. In cases that display diminished yields, the balance of the material consists of the protonated product 72 (E = H). 

Submicroliter samples are needed for the capillary on-column injection technique. To counter a general scepticism following the development of this injection technique, excellent quantitative results have been reported [42). The major advantages of this injection procedure seem to be in minimizing thermal decomposition of labile compounds as well as the lack of sample discrimination toward the later eluting components. This latter problem, observed frequently with the vaporizing injector... 

Apparently the rate of radical recombination is high so that under anaerobic conditions the compound is stable, but in air, radical trapping by oxygen promotes net decomposition. The unusual lability of neopentylcobalamin towards spontaneous mode II cleavage was attributed to steric compression around the coordinated carbon in 6 coordinate complexes as evidenced by the fact that the compound is stable in acid (even in the presence of O2) where the pendant ligand is dissociated and protonated (see Section 5.3). 

The presence of a chelate ring, in general, should increase the thermal stability of metal complexes towards decomposition through a decrease in kinetic lability. Such trends have been observed in, for example, the solid state isomerization of platinum(II) complexes containing monodentate triphenylphosphine and bidentate dppm ligands. A study thermally-... 

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