OF Ligand Exchanges

NMR techniques can be useful for the determination of the site of action of a metal complex, of its binding constant, and rate data, particularly the rate of exchange of ligands. There are two types of experiments that can be performed. 

A coordination isomer involves the exchange of ligands between a complex cation and a complex anion of the same compound, forming another complex cation and complex anion. 

Thus, after the formation of the 7t-allyl complex 397 from the corresponding allyl trifluoroacetate, an exchange of ligand with triphenylphosphine generates 398. The formation of the phosphine-Pd complex 398 appears to be the key to successful cyclization, because the complex 397 failed to cyclize. 

The drawback of methods which are based on observation of ligand resonances is that they rely on a significant exchange of ligand between bound state and free state during... 

If the flexion barriers of monocentric skeletons are such that the exchange of ligands occurs only by certain mechanisms, the number of... 

In this case one may assume that the charge transfer takes place as soon as the two reagents collide, without the occurrence of any exchange of ligands (which would imply breaking of old or formation of new bonds in the reaction intermediate). 

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

Once we open the system to allow exchange of ligands between the sites and the reservoir, the number of occupancy states of our system is not Q) (or 6 in the case of Fig. 1.1), but 2 " (or 2 = 16 as in Fig. 1.2). This is so because any site can be either empty or occupied, i.e., 2 states for each site, hence 2 " states for the m sites. Clearly, in an open system these 2 " configurations are not equally probable. For calculating the probabilities of the various events statistical mechanics provides a general recipe which differs from the classical method used above. The latter is applicable only when there are Q equally probable events (say, six outcomes of casting a die with probability 1/6 for each outcome). 

Because the easy exchange of ligands and alkyl groups between aluminiums, activation of an aluminium dichloride or diaUcyl complex with MAO leads to complex mixtures and to results that are difficult to interpret. 

A very interesting ligand exchange at the metal atoms is observed in compounds of types 35-38. The two metal atoms within the molecule are in competition for the two nitrogen atoms, of which only one is attainable for steric reasons. As a consequence, one aluminum atom becomes electronically unsaturated and has to participate in a three-center two-electron bond (see also Fig. 2). This bonding situation can be rapidly inverted by an exchange of ligands as shown in Eq. (36) for compound 35 (64). 

If one or more ligands L in laige excess interact with a metal ion in a metal complex CM in the presence of free metal ions M in solution, then the exchange of ligand L among three sites should be considered. A typical case is when L is a coordinating solvent molecule, e.g. water. The molar fraction of water nuclei is given by... 

It is well known that the chiral resolution of these CSPs occurred as a result of the exchange of ligands and enantiomers on the same metal ion. Therefore, these CSPs are suitable only for those racemates which can coordinate with the metal ion. Therefore, racemates like amino acids, amines, and hydroxy acids have been resolved successliilly by the ligand-exchange process. As mentioned earlier, either the individual chiral ligand or one complexed with a metal ion is bonded onto silica gel support. Therefore, in the case of the first type of CSP, the metal ion is used in the mobile phase no metal ion is required in the mobile phase in the latter case. 

Racemization reactions due to successive exchange of ligand roles and successive Walden inversions were discussed in Sections 3.1 and 3.2. The decarbonylation reaction seems to be stereospecific, yet, as mentioned in Section 3.3, subsequent to CO elimination there is a racemization of the decarbonylation products. 

It is necessary to consider that the competitive reactions are characteristic also for ligands having two or more hard and soft donor centers. The first of them are NCO, NO2, NO., and other anions [2,7,44], The synthesis of cyanate complexes is mainly carried out by the method of immediate interaction of ligands and metal cyanates [(3.274), X - O] or by exchange of ligands, for example (4.16) [44] ... 

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