Carbonylation replacement

Two potential problems (opportunities ) which may be encountered are (i) redox condensation, whereby carbonylates of higher nuclearity may be formed (Figure 3.5a,d,e) effectively, the reduced carbonylate replaces a carbonyl ligand on combination with a second equivalent of the original neutral carbonyl (ii) over-reduction, whereby more than two electrons may be added, e.g. in the formation of Na4[Cr(CO)J or Na3[Mn(CO)4] (Figure 3.5b). These problems are controlled by judicious (or empirical) choice of solvent and reductant. 

Carbonyl replacement dominates the observed photoreactions of Co complexes, and with CO-like ligands, total substitution is possible. Table 1 lists examples. The complex CofCOjNO is unusual in that the primary photoprocess is a linear to bent NO transformation, followed by an associative displacement of CO from this shortlived intermediate. Photolysis in CHClj leads to incorporation of chloride in the complex if PPhj is present, then Co(PPh3)2Cl2 is formed " if NO is present then [Co(NO)2C1 j is generated Trialkylsilane exchange in RjSiCo(CO) (Table 1) proceeds via oxidative addition of RjSiH to the intermediate formed by CO loss, followed by reductive elimination of R SiH. This is true for surface-confined analogues as well d. 

Substitution reactions of Tc2(CO)in]° with PF3 were observed under ihermal conditions at 1 lO C and under photolytic conditions at room temperature using a 450 W medium-pressure mercury lamp. Thermal reactions produced compounds with up to six of the carbonyls replaced by trifluorophosphinc, photolytic reactions yielded prod-... 

Nucleophilic attack on carbonyl Replacement of carbonyl oxygen Rate and pH chi 2 ... 

The carbonyl replacement reaction between Co(NO)(CO)3 and a variety of ligands L (phosphines, phosphites, pyridines, and isonitriles) to give Co(NO)(CO)2L has been studied. The reactions are fast with phosphines... 

R = Bu ) has been prepared from tetra-t-butylcyclotetraphosphine by heating with excess sulphur, and (155 R = Ph) from bis(trifluoromethylthio)phenyl-phosphine and sulphur. These anhydrides, particularly (155 R=p-anisyl), have been exploited for the preparation of thiocarbonyl groups from the corresponding carbonyls (replacing the more obnoxious P2S5) e.g., thioketones, ... 

This is quite an unimpressive minimum well. However, in considering the contribution of HBing between two H0I groups in biomolecules, we have to deal with two groups that have fixed orientation relative to each other and that are attached to some carrier or backbone. The situation is the same as in Fig. 7.21 but with H0I groups, say hydroxyl or carbonyl, replacing the methyl group. Therefore, we should examine the conditional contribution of the solvent-induced interaction between two H0I at some fixed relative orientation. 

Cp Ir(H)2(CO)] is probably an intermediate under these conditions. Study of the reductive elimation of hydrocarbons from [Cp Rh(H)(R)L] revealed that the ease of elimination is influenced by the nature of the phosphine, L, in terms of both steric and electronic factors.A lack of isotope scrambling in frozen CD4 matrices supported the mechanism of simple H2 elimination during irradition of [CpIr(H)2(CO)]. ° On the other hand, carbonyl replacement by PPh3 in [CpRh(CO)2] showed a l iear relation between quantum efficiency and phosphine concentrations. This suggests that an associative process dominates with this nucleophile. 

For the Group VI carbonyls replacement of 2,3 or 4 carbon monoxides by olefin ligands has been achieved. However, the substitution of four carbon monoxide ligands has only been formd with chelating ligands metal-tricarbonyl systems are usually markedly resistant to substitution. In the... 

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