Polarisation of the Ligand

We could imagine two types of nucleophilic attack upon a ligand. The first involves the formation of a new bond to the incoming nucleophile and the associated loss of some other leaving group. This corresponds to nucleophilic displacement. The second type of reaction does not involve the loss of any other species and corresponds to an addition reaction. There are a number of ways in which the polarisation of the ligand can enhance a... 

The balance between stabilisation and activation of the imine towards hydrolysis depends on the relative polarisation of the ligand and the back-donation from the metal, as discussed in Chapter 2. It is very difficult to successfully predict the overall stabilisation or destabilisation of a given imine towards hydrolysis in the presence of a given metal ion. Some imines are stabilised by co-ordination to copper(n), whereas others are destabilised (Fig. 4-23). 

The principal role of the metal ion is to increase the polarisation of the ligand prior to attack by the nucleophile, but a very important secondary role involves the stabilisation of anionic leaving groups. 

The precise sequence of events depends upon the combination of ligands and metal centres involved, but the key step involves a C-H bond-breaking reaction. The reaction may be viewed as a consequence of metal ion polarisation of the ligand increasing the acidity of the relevant C-H bond. Loss of a proton yields a carbanion, which undergoes an electron transfer reaction with the metal centre to yield a radical and lower oxidation state metal ion (free or co-ordinated). It must be emphasised that this is purely a formal view of the reactions. 

We have also used a non-radiometric-binding approach based on fluorescence polarisation [29], where a fluorescent label is used in place of a radiolabel. As the fluorescently tagged oxytocin binds to the receptor, its rotational velocity is reduced and the polarisation of the fluorophore increases. The displacement of the ligand may be measured by a decrease in polarisation. 

If the metal ion to which a ligand is co-ordinated is in a non-zero oxidation state, it will exert an electrostatic effect upon the bonding electrons of the ligand. This will result in the induction of a net permanent dipole in the ligand, with any associated chemical and physical effects. Even zero-oxidation state metal centres may induce a polarisation in the ligand through electronegativity or induced dipole-dipole effects. 

The introduction of 7i-bonding interactions between the metal and the ligand results in a metal-to-ligand or ligand-to-metal transfer of electron density. This occurs in accord with the electroneutrality principle, and in many cases, it opposes the polarisation effects of the metal ion. Furthermore, the effect will be expressed in orbitals possessing rather specific symmetry properties which may play important roles in the reactivity of the ligands. 

A metal ion might play a number of roles in modifying the behaviour of co-ordinated electrophiles. Co-ordination of a donor atom X to the metal polarises the C-X bond and increases the electrophilic character of the carbon. In the case of an unsaturated centre this polarisation may be countered or even completely reversed by back-donation from filled (d) orbitals on the metal into vacant 7t -(or similar) orbitals of the ligand. The X- group generated after nucleophilic attack on the carbon atom may also be stabilised by co-ordi-... 

However, the reaction requires only a general acid catalyst rather than the specific acid catalyst H+, and the corresponding reactions of the soft thioether may be better mediated by softer Lewis acids such as Cu+, Ag+, Hg2+, Pd2+, Pt2+ or Au3+. In many cases the aqua-ted metal ion is the most convenient Lewis acid, but in the case of some metals, particularly the second and third row transition metal ions, the aqua ions are not isolable and other complexes (particularly those with chloride ligands) are equally effective. The role of these softer metal ions as Lewis acids is two-fold. Firstly, the sulfur is co-ordinated to the metal, which increases the polarisation of the C-S bond and enhances the electrophilic character of the carbon, and, secondly, the thiol (or thiolate) leaving group is stabilised by co-ordination (Fig. 4-39). 

One difference between the Suzuki mechanism and that of the Stille Coupling is that the boronic acid must be activated, for example with base. This activation of the boron atom enhances the polarisation of the organic ligand, and facilitates transmetallation. If starting materials are substituted with base labile groups (for example esters), powdered KF effects this activation while leaving base labile groups unaffected. 

The bromides and iodides are made by direct combination of the elements. Some are coloured TiBr4 is yellow and TH4 red-brown, in accordance with the position of the ligands in the spectrochemical series (p. 134). They are solids of low m.p. the crystals have a cubic lattice and contain tetrahedra molecules. Some bromo-complexes have been made, for instance NH4)2TiBrg.2H20, but they are much less stable than the fluoro-compounds iodo-complexes are unknown the stability falls rapidly with the more easily polarisable halogens. 

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