Ligands common monodentate

We see that only complexes with formation constants of the order of 106 M-1 or more will lead to titration curves with a sufficiently steep change in pL near the equivalence point (at CM VM / CL VL = 1) to be useful for volumetric analysis. None of the common monodentate ligands, such as the halide anions (Cl-, Br , I-) or the pseudohalides (CN , SCN-, N3 ), form such strong complexes, nor do the carboxylic acid anions (such as acetate) or ammonia (NH3). However, in section 5.2 we will encounter special ligands, the chelates, that do form sufficiently strong 1 1 complexes. 

Platinum co-ordination complexes share the common formula PtA2X2 with only the a s-isomers displaying an anti-tumour activity. The active X ligands are monodentate anions of intermediate leaving ability, whereas the amine A ligands influence... 

Copper complexes of azo dyes are used widely in both reactive dyes (Chapter 8) and direct dyes (Chapter 7) for cellulosic fibres. In these dyes, the copper complexes adopt four-coordinate square planar geometry, with the three coordinating sites of the dye occupying corners of the square and the fourth occupied by a monodentate ligand, commonly water (Figure 3.6). The most important cobalt and chromium complexes of azo dyes adopt six-coordinate octahedral geometry, with the six positions occupied by coordination with two... 

Common Monodentate, Multidentate, Bridging, and Ambidentate Ligands... 

Note - In inorganic chemistry, the most common monodentate L ligands are water, amines, ethers, thiols and sulfides (Chap. 1.1). They are all good donors, because they are not n acceptors. Thus, they are weakly bound to metals, and, as such, also frequently used in organometallic complexes for facile substitution chemistry (see Chap. 5) and catalysis (see Parts IV and V). 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

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. 

It should be noted that the Grob fragmentation reaction and the reductive cyclization (homoallylation) discussed in this section involve the same oxanickellacyclopentane 66 as a common intermediate (Scheme 17). The reversibility of these C - C bond cleavage reaction and C - C bond formation reaction is also supported by the isolation and characterization (by X-ray analysis) of an oxanickellacyclopentane-like 66 (without a tether), which is prepared from a stoichiometric amount of Ni(cod)2, a diene, an aldehyde, and a monodentate phosphine ligand [41]. 

Pyrazolate as a ligand can exhibit various coordination modes, inter alia, monodentate, exo-bidentate, and endo-bidentate. Several review articles have been published in the field.556-559 The use of pyrazole in the modeling of biological systems was also reviewed.560,561 The most common application of pyrazole ligands is to use its exo-bidentate coordination mode linking two metal centers that may be identical or different (compare Section 6.3.4.12.6). Polypyrazolylborates and related ligands are covered in the subsequent section. 

General Structural Features. The general structure of halfsandwich ruthenium(II)-arene complexes is shown in Fig. 12. The structural, stereochemical and electronic features of metal-arene complexes have been discussed (63). A typical piano-stool geometry consists of an rj6-arene occupying three coordination sites of the pseudo-octahedral complex, leaving the three legs X, Y, and Z available for coordination. The sites X and Y can be taken up by two monodentate ligands, but are more commonly... 

Fig. 12. (a) General structure of the half-sandwich, piano-stool ruthenium—arene complexes (b) X and Y are commonly occupied by a bidentate ligand L giving a monofunctional complex (c) tethering of a monodentate ligand to the arene results in a bifunctional complex. 

The use of monodentate phosphoramidites in enantioselective hydrogenation was first reported in 2000, together with reports on the use of phosphites and phospho-nites [15]. Phosphoramidites are prepared in a variety of ways, but the most common route is the treatment of a diol with PC13, followed by addition of an amine [60, 61]. MonoPhos (29a), the first reported phosphoramidite used as a ligand, is prepared from BINOL and HMPT in toluene [62]. Phosphoramidites, especially... 

Breakthroughs that took place around the year 2000 have shown, in contrast to the common view, that indeed chiral monodentate phosphorus ligands can also lead to high enantioselectivities in a number of asymmetric hydrogenations. In the years following, monophosphines, monophos-phonites, monophosphoramidites, and monophosphites have been successfully used in the enantioselective hydrogenation of a-dehydroamino acids and itaconic acid derivatives [25],... 

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