Type Ligands

It is probably true that almost every coneeivable molecule, atom or ion could act as a ligand under some circumstance or other. However, certain types of ligands are commonly encountered, and it is these, together with the vocabulary which they generate, that we introduce at this stage. 

The majority of ligands are either neutral or anionic. Those which coordinate to a metal ion through a single atom are described as monodentate or unidentate. Examples of such ligands which we have encountered thus far include water, ammonia and chloride. A more extensive listing of common ligands is found in Table 1-3. We stress at this point that there is no difference in kind between the interactions of a metal centre with either neutral or anionic ligands. 

A number of general features in Table 1-3 is apparent. Complexes may be cationic, neutral or anionic. Ligands may be simple monatomic ions, or larger molecules or ions. Many ligands are found as related neutral and anionic species (for example, water, hydroxide and oxide). Complexes may contain all of the same type of ligand, in which case they are termed homoleptic, or they may contain a variety of ligand types, whereby they are described as heteroleptic. Some ligands such as nitrite or thiocyanate can coordinate to a metal ion in more than one way. This is described as ambidentate behaviour. In such cases, we commonly indicate 

H2NCH2CH2NHCH2CH2NH2 Diethylenetriamine, 1,4,7-triazaheptane, bis(2-aminoethyl)amine, dien. 

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Central unit Ligand Co-ordination number Ligand type Complex ion Shape... 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

The influence of flow rates of eluent and sample solution, amount of ligand, types and least amount of eluent for elution of Cobalt ion from column were investigated. 

In the absence of specific interactions of the receptor - ligand type the change in the Helmholtz free energy (AFadj due to the process of adsorption is AFads = yps - ypi - Ysi, where Yps, YPi and ys, are the protein-solid, protein-liquid and solid-liquid interfacial tensions, respectively [5], It is apparent from this equation that the free energy of adsorption of a protein onto a surface should depend not only of the surface tension of the adhering protein molecules and the substrate material but also on the surface tension of the suspending liquid. Two different situations are possible. 

Notice the loose use of the term octahedral to describe six-coordinate complexes which are based upon an octahedral geometry, but which, by virtue of the presence of different ligand types, are of lower symmetry than Oh. This is a common usage which should give rise to no difficulties. Note also how introduction of chelating... 

One of the features of coordination chemistry which we try to explain is the preference of certain metal ions for certain ligand types. In this section, we briefly discuss the models which have been developed to rationalize observed patterns of ligand recognition. We preface our discussion by noting, however, that the models... 

So far, only for the first two of the six ligand types outlined in Fig. 10 have oligomeric structures with boron been reported. This may be in part due to the relatively small number of publications related to this field and the fact that a systematic attempt to prepare boron macrocycles with these ligands has not been realized so far. 

While inference has been made in a few cases as to the order of addition of ligands in prethermal reactions, no generalization can yet be made. When only one type of ligand is present, it is clear that the same ligand type may... 

It is important to emphasize that this lattice database is highly idealized compared to real databases. Unlike the lattice database, real databases cannot be treated as thermodynamic ensembles of protein-ligand complexes equilibrated at room temperature [33,34]. Two of the more straightforward reasons are mentioned here. First, real databases are inherently biased toward strong binders (K < 10 pM), because weak binders are difficult to crystallize and of lesser interest. Second, as mentioned above, real databases are not composed of a representative selection of proteins and ligands, and their compositions are biased toward peptide and peptidomimetic inhibitors and certain protein superfamilies. In contrast, because only one protein and four ligand types are used, the lattice database should have representative ligand compositions. 

On the other hand, consider the probability that a particular ligand type is making a certain number of protein contacts. This is given by... 

Some general considerations governing the nature of selective enantiomeric interactions for both gas and liquid chromatographic phases (at least of the bonded monomeric ligand type) have been forthcoming [721,742,754,756,781,782,790). It is generally assumed that three points of simultaneous interaction at least one of which must be stereochemically controlled, are required to distinguish the chirality of a molecule. These... 

Much progress has been made on regioselective hydroformylation of terminal alkenes in favor of the linear product. In particular bidentate phosphine or phosphite ligands, which have a natural bite angle 9 of about 110°, will favor the linear product. The most successful ligand types are BISBI [49, 50], BIPHEPHOS [51,52], and XANTPHOS systems (Scheme 8) [53]. 

To better understand copper-dioxygen coordination chemistry with varying ligand type, Karlin and co-workers synthesized and investigated spectroscopic and redox properties of many complexes (218) (t= 1.005) and (219) (r = 0.19), (220) (t = 0.50),209 (221) (r = 0.19) and (222)... 

There is significant interest in zinc sulfide, selenide, and oxide materials and while extensive discussion is not appropriate here, a number of novel complexes that have been developed for their deposition characteristics of these important semiconductors will be highlighted in the context of the ligand types. Zinc has also been used in supramolecular building blocks as a structural element, much as in zinc finger proteins. In these cases the lack of redox chemistry is... 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

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