Reactivity ligand-centred reactions

It is useful to consider the reactions of carbonyl metallates separately, since their reactivity is generally concerned with the nucleophilic metal centre and will be discussed below. Simple ligand substitution reactions have already been discussed above, as have redox processes that provide access to carbonyl metallates through reduction of the metal centre. These redox or ligand addition/elimination processes are in principle no different from those encountered for classical ligands. We will now consider reactions in which the carbonyl ligand itself enters directly into the reaction and emerges transformed. 

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

When a chiral metal complex forms a complex with a prochiral alkene , either because it contains a chiral ligand or a chiral metal centre, the resulting complex is a diastereomer. Thus, a mixture of diastereomers can form when the chiral complex coordinates to both faces of the alkene. As usual, these diastereomers have different properties and can be separated. Or, more interestingly, in the catalytic reactions below, the two diastereomers are formed in different amounts and their reactivities are different as well. 

The coordination chemistry of silver has historically been centred on the reaction of silver(I) ions with N-donor ligands and halides. However, an extensive chemistry now exists for P- and S-donor ligands, whilst for O-donor ligands only weak complexes are generally formed and they have been studied in much less detail. Based on the reactivity and stability of its coordination complexes, the silver(I) ion has been characterized as a class B or soft acid, for which the following stability order is observed N P>As>Sb 0 S Se Te FComparative studies between ligands with these donor atoms allowed the relative stability of silver bonds to be determined as P > S > N > O. 

A slightly different pattern of reactivity is seen when 3.8 interacts with antimony(m) fluoride in methanol. In this case, the product contains a ligand derived from the hemia-cetal, but the hydroxy group is deprotonated and co-ordinated to the metal centre, to give an N,iV,0-bonded anionic ligand (Fig. 3-17). It is not known whether the co-ordination of the oxygen to the metal centre occurs after the hydration reaction (in which case we are... 

Sometimes this deactivation is so great that co-ordinated amines are non-nucleophilic. This is particularly likely when the ligand is co-ordinated to a non-labile metal centre. However, even in these cases, all is not lost. We may also use the enhanced acidity of ligands co-ordinated to a metal centre to generate reactive nucleophiles which would not otherwise be readily accessible. For example, nickel(n) complexes of deprotonated diamines may be prepared, and react with dialkylating agents to yield macrocyclic complexes (Fig. 6-10). To clarify this, consider the reaction in Fig. 6-10 in a little more detail. The amine 6.14 is reactive and unselective, and does not give the desired macrocycle upon reaction with the ditosylate. Deprotonation of the amine under mild conditions is not pos-... 

---