Subject monodentate

Finally, if there could be a way in which in water selective ri Jt-coordination to the carbonyl group of an a,P-imsatLirated ketone can be achieved, this would be a breakthrough, since it would subject monodentate reactants to catalysis by hard Lewis acids ". ... 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

Each species (99), (100), (101), (102), or (108) has been subjected to a wide variety of substitution processes of the ylide hgand by other anionic hgands, or of the X ligand by other anionic or neutral L ligands, acting as monodentate, chelate or bridge. This results in a plethora of complexes with very diverse structural features, whose complete description falls outside the remit of this chapter. However, there are some noteworthy processes which merit mentioning here. 

Chelate complexes could only be prepared in the case of platinum(II) as the metal ion, while the group V atom alone acted as a donor toward palladium(II) and mercury(II). The coordinated olefin in the chelate complexes was found to be readily displaced by monodentate ligands such as tertiary arsines, -toluidine and the thiocyanate ion. It was suggested by these workers that chelation would take place more readily if the olefinic phosphine or arsine were subject to greater steric restrictions than was the pentenyl ligand. 

DiaUcyl as well as diaryl tellurides behave as monodentate ligands. Consistent with their soft character, their best explored group of complexes involves Pd(II) and Pt(II) centers. A variety of techniques have been applied to the characterization of their solid state and solution stractures. In such square-planar complexes, [MX2(TeR2)2] (M = Pd, Pt X = Cl, Br, I), cis - trans isomerization, intramolecular ligand exchange and tellurium inversion processes have all been observed by detailed far-infrared and variable-temperature NMR studies (see Refs. 14,15,17,3 5 for a more detailed review of the subject). 

The simplest group VA carbonyls subjected to photochemical study are the 18-valence-electron [M(CO)j] (M = V, Nb, Ta). Irradiation induces carbonyl loss, and in the presence of monodentate ligands simple monosubstituted [M(CO)jL] products result ... 

One more class of thallium(III) compounds has been subjected to structural investigations, namely the compounds of the type TKpoly-dentate ligandlX, where the polydentate ligands were EDTA (ethylene-diaminetetraacetate), DTPA (diethylenetriaminepentaacetate), TPEN (tetrakis-(2-pyridylmethyl)-ethylenediamine), NTA (nitrilotriacetate), bipy (bipyridine), terpy (2,2" 6, 2 terpyridine), pent-en (N,N,N, N -tet-rakis-(2-aminoethyl)-ethylenediamine), andX = OH, H2O, CN", N03 (106, 109, 94). In these compounds, thallium(III) is usually hepta- or even octa-coordinated, in contrast to monodentate ligands, where the... 

Figure 2 Mechanism of urease. This proposed mechanism features binding of urea through its carbonyl group to one of the Ni ions, making the carbon subject to nucleophilic attack by the bridging hydroxide leading to liberation of NH2, which is protonated by to form ammonia, and carbamate, which hydrolyzes to another mole of ammonia and carbonic acid. The same mechanism with proton transfer from water (instead of histidine) to ammonia is another possibility. Another possibility would be urea binding in a monodentate manner with a terminal hydroxide on Ni2 as the nucleophile...

A classical method for the preparation of enantiopure compounds is the resolution of racemate. However, it is much more effective to use the selective synthesis of the desired enantiopure substance via enantioselective approach. Stereoselective methods of synthesis have been widely developed in organic chemistry. The method of asymmetric synthesis has been known since the nineteenth century and asymmetric catalysis has witnessed an enormous amount of development in recent decades as shown in Chapter 3. In contrast, the asymmetric synthesis of coordination compounds has only recently become a subject of systematic investigation. This is no doubt related to the fact that the chirality of coordination compounds is a much more complex phenomenon than that of organic compounds, because of higher coordination and the multitude of possible central atoms. Furthermore, while in organic chemistry the chiral tetrahedral carbon centres can be prepared without racemization, in contrast T-4 metal centres are very often labile. In fact it is even difficult to prepare compounds with a metal centre coordinated to four different monodentate ligands, and thus the possibility of obtaining one enantiomer is excluded in most cases. 

Insoluble polymers based on bidentate M-S dithiolene and M-O dioxalato ligation have been the subject of numerous studies. Although these materials are, in general, poorly characterized they have, in some cases, been shovm to exhibit promising conductive properties [100, 101]. Polymers with M-S and M-N/O ligation have also been prepared on surfaces [102, 103]. Monodentate M-N ligation has been used to prepare remarkable nanoscopic 3-D assemblies 7.54 by means of rigid, multifunctional precursors (Eq. 7.14) [104]. 

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