Predisposed ligands

Therefore, another strategy was developed, based on the induced-fit concept, which uses flexible receptors in order to optimize the interactions between the donor atoms and the metal ion. In fact, the coordination environment is built upon complexation thanks to the flexibility introduced into the complexation agent, which is now termed predisposed ligand . These receptors are either large macrocycles able to wrap around the guest or small macrocycles fitted with pendant arms. The latter approach has proved to be very successful, particularly with calixarene (Asfari et al., 2001) and cyclen (1,4,7,10-tetraaza-dodecane) (Lukes et al., 2001) derivatives. 

This is expected to be favoured for metal-centred excited states for example, in d-d states of d or d complexes, where excitation often involves promotion of an electron from an essentially non-bonding orbital to one with appreciable sigma antibonding M-L character (e.g. in CrfNHalsCl Eq. 3). The net effect is lengthening of the M-L bond, which predisposes the complex to dissociation or associative substitution. The incoming ligand is often the solvent (e.g. as in Eq. 3) or counterion of an ion pair (Eq. 4). 

It is not easy to explain Irving and Williams rule that complex formation constants show a maximum for Cu(II) between Ni(II) and Zn(II) as a consequence of ligand field stabilization. It is true that multidentate ligands sterically predisposed to low ratios of tetragonality (such as 1.35 for ethylenediaminetetra-acetate) also show this effect to a less pronounced extent. Thus, log/f j can be compared6 22,23 with bidentate en and tridentate den ... 

Cholecytokinin ligands - agonists of the central CCK receptors cause anxiety and precipitate panic attacks in predisposed individuals. Two types of CCK receptors have been identified, CCK-A and -B (from the alimentary tract and brain respectively), both of which occur in the mammalian brain. CCK-B agonists initiate anxiety while the antagonists are anxiolytic in both experimental and clinical situations. So far the poor bioavailability and side effects have limited their clinical development. 

The excellent control observed with the P,S- and certain N,S-ligands is believed to be mainly electronic in origin [85-88, 90-92, 94, 95]. When a tt-allyl system possesses two different coordinating atoms, the nucleophilic attack is expected to occur trans to the better Jt-acceptor, since the electronic density of the allylic system is lowest at this position. In these cases, the phosphorus and the nitrogen atoms are better Jt-acceptors than the sulfur which is a good donor but weak acceptor. In some cases, the sulfur atom is considered to be the better acceptor [90]. In addition, it has been proposed that the selectivity arises from subtle steric interactions that predispose attack on the allyl unit of the reaction intermediate with a preferred reaction trajectory [93]. 

The use of transition metals or transition metal clusters to act as nodes for the modular self-assembly of diamondoid networks that are sustained by coordinate covalent bonds is also well established. Such architectures are of more than aesthetic appeal. Indeed, such structures have resulted in a class of compound with very interesting bulk and functional properties. Metal-organic diamondoid structures in which the spacer moiety has no center of inversion are predisposed to generate polar networks since there would not be any inherent center of inversion. Pyridine-4-carboxylic acid is such a ligand and bis(isonicotinato)zinc exists as a three-fold diamondoid structure that is thermally stable and inherently polar.33... 

They are predisposed to form acentric networks since there is no inherent center of inversion at a tetrahedral node. Indeed, an odd level of interpenetration and an unsymmetrical ligand will definitely generate a structure that exhibits polarity.22... 

In this case, will be used to exemplify metals strongly predisposed toward precipitation as well as complexation with humus. Like Cu ", AP complexes energetically with catechol-type and other polyphenolic ligands. The stability constant, ATail for the solution reaction ... 

Given the immense number of metal-ligand pairs possible in systems as complex as soils, the question arises How do we decide in which cases a ternary complex might influence metal solubility It is reasonable to expect that those particular cations and anions that are predisposed to form ion pairs in solution will display this same tendency to pair on adsorptive surfaces, forming ternary complexes. This is a statement of the general principle that ... 

As with all such palladium-catalyzed carbon-heteroatom bond-forming chemistry, the ancillary ligand(s) (i.e., L often featuring phosphine or A-heterocyclic carbene donors " employed have a direct influence over the course of the elementary transformations. Electron-rich and sterically demanding ligands promote the formation of low-coordinate compounds of type A that are predisposed to undergo Ar—X... 

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