Ligand, monodentate

Recall that all six positions in an octahedron are equivalent. Therefore, when one of the monodentate A ligands in a coordination compound of formula MAg is replaced with a different ligand B, only one possible configuration exists for the resulting MA5B complex. (There may be different ways to draw them, but all such 

Fac (or cis) and mer (or trans) isomers of tri-aquatrichlorochromium(III). (o) The fac (or cis) isomer with the triangular face of chloride ligands outlined (dashed lines), (b) The mer (or trans) isomer with the chlorides along half the meridian outlined (dashed lines). Both geometric isomers possess an internal mirror plane (M) and are nonchiral. 

The five geometric isomers of MA2B2C2. (a) and (b) have the two A ligands in the trans positions, whereas (e), (d), and (e) have them in the cis positions, (a) to (d) have internal mirror planes, but the all-cis isomer (e) has no mirror 

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], 

A DSM s Monophos sample-kit of ligands as developed by Feringa, de Vries, and co-workers is commercially available from Strem Chemicals (as well as various kits of Solvias ligands, such as Josiphos). 

The BINOL substituent seems to be crucial firstly the related phosphites and phosphinites function just as well as the amidites, and secondly the bidentate BINOL based phosphorusamidite performs reasonably well compared to other groups. So far, all other substituents at oxygen, such as TADDOL, give low e.e. s. As an indication that BINOL amidites are not 

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The chemistry of complexes having achiral ligands is based solely on the geometrical arrangement on titanium. Optically active alcohols are the most favored monodentate ligands. Cyclopentadienyl is also well suited for chiral modification of titanium complexes. 

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

Perchlorate ion established as a monodentate ligand (to Co) by X-ray crystallography, following earlier spectrosopic and conductimetric indications of coordination (1961)... 

Here the ligand (L) can be either a neutral molecule or a charged ion, and successive replacement of water molecules by other ligand groups can occur until the complex ML, is formed n is the coordination number of the metal ion and represents the maximum number of monodentate ligands that can be bound to it. 

The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium. Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria, in which, for convenience, coordinated water molecules are not shown ... 

The term chelate effect refers to the fact that a chelated complex, i.e. one formed by a bidentate or a multidenate ligand, is more stable than the corresponding complex with monodentate ligands the greater the number of points of attachment of ligand to the metal ion, the greater the stability of... 

Monochromator 663, 791 Monodentate ligand 51 Morphine (and codeine) D. of (fu) 740 Mortar agate, 155 mullite, 155 percussion, 155 synthetic sapphire, 155 Muffle furnace 97 Multielement analysis 174, 184, 775 Multiple range indicators 268 Murexide 316... 

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. 

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Table 1-3. Some typical monodentate ligands and representative complexes that they form.

There is one striking group of exceptions to the otherwise almost unbroken success of Kepert s approach. No model predicated solely upon the repulsions between monodentate ligands (or between bonds) can account for the planarity of some four-coordinate complexes. Yet hundreds of planar (f complexes like [Ni(CN)4] or [PtCl4] are known. Clearly, Kepert s model is to be augmented and we discuss this matter further in Chapter 7. 

When two or more donor atoms from the same ligand are coordinated to a single metal centre, the ligand is said to be chelating. It is a general observation that chelated complexes of polydentate ligands are always more thermodynamically stable than those of the same metal with an equivalent number of comparable monodentate ligands. That is to say, the equilibrium... 

Figure 8-2. Schematic representation for the formation of a complex. In the case of the monodentate ligands, there is a greater unfavourable nitrogen-nitrogen repulsion involved in bringing the ligands together.

In forming the chelate complex, there is a high probability of the seeond donor atom Y forming a bond to the metal whereas, with monodentate ligands, the probability is much lower. In other words, once the first M-L bond is formed, the second donor atom is held close to the position required for the formation of the second bond. 

In more mathematical language, the favourable entropy term is associated with the release of a large number of monodentate ligands upon the formation of the chelate. 

Both complexes have six monodentate ligands, so both are octahedral. 

L = neutral monodentate ligand L-L = neutral bidentate ligand X = halide ligand Ar = aromatic ligand Cp = cyclopentadiene. 

The Rh-catalysed asymmetric hydrosilylation of prochiral ketones has been studied with complexes bearing monodentate or heteroatom functionalised NHC ligands. For example, complexes of the type [RhCl(l,5-cod)(NHC)] and [RhL(l,5-cod)(NHC)][SbFg ], 70, where L = isoquinoline, 3,5-lutidine and NHC are the chiral monodentate ligands 71 (Fig. 2.11). 

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