Ligands, in complexes

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-t/ transition-metal complexes. The molecular stmcture of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = is shown in Figure 1. The... 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

Heterocycles as ligands in complex catalysts for asymmetric cyclopropanation 98T7919. 

Mutual influence of ligands in complexes. Y. N. Kukushkin, Russ. Chem. Rev. (Engl. Transl), 1974,43, 804-820 (166). 

The S2O ligand in complexes is coordinated to the metal atom through both sulfur atoms which causes an increase of the SS bond length. Such complexes are also accessible by the reaction of sulfinyhmido complexes with H2S [52]. 

Several studies have been undertaken in an attempt to understand factors that affect the susceptibility of a complex to reductive elimination. These include investigations on Me-Pd-CNC pincer complexes [39] studies on the influence of the geometry of the complexes [40] the impact of A-substituents [41] and a study on the influence of chelating spectator ligands in complexes of the type [PdMe(NHC) (P-P)]BF [42]. 

Pd complexes 9-12 were tested for their catalytic behavior in the asymmetric Heck reaction involving the phenylation of 2,3-dihydrofuran (Scheme 3). The results are summarized in Table 2. The two isomeric products of 2-phenyl-2,5-dihydrofuran are formed with varying yields from 80% to 0%. The obtained ee s are high. Complex 12 is shown to be catalytically inactive. The lack of catalysis in complex 12 is rationalized by differences in the steric requirements between the diphenylphosphinites 1-3 (cone angle >140°) and the more sterically hindered cyclohexyl-phosphinite 4 (cone angle >170°) and the resulting stereochemistry on the Pd center. The ligands in complex 12 adopt a... 

We have not yet addressed the important topic of absorption by the ligands in complexes. For many types of complexes, this type of spectral study (usually infrared spectroscopy) yields useful information regarding the structure and details of the bonding in the complexes. This topic will be discussed later in connection with several types of complexes containing specific ligands (e.g., CO, CN-, N02-, and olefins). 

This procedure has been used successfully to determine the composition of many complexes in solution. It is possible to extend this method to cases where more than one complex is formed but the application is quite difficult. Like the logarithmic method, Job s method can be applied to other cases of molecular interaction and is not limited to the formation of coordination compounds. Both methods are based on the assumption that one complex is dominant in the equilibrium mixture. Numerous other methods for determining the number of metal ions and ligands in complexes have been devised, but they are beyond the introduction to the topic presented here. 

The binding energies in Table 4.44 show the expected strong preference for anionic over neutral ligands in complexes of the metal cation. However, the geometries and other properties of these complexes reflect strong covalency effects (albeit enhanced by net ionic attraction) that will principally be considered. 

Following Wilkinson s discovery of [RhCl(PPh3)3] as an homogeneous hydrogenation catalyst for unhindered alkenes [14b, 35], and the development of methods to prepare chiral phosphines by Mislow [36] and Horner [37], Knowles [38] and Horner [15, 39] each showed that, with the use of optically active tertiary phosphines as ligands in complexes of rhodium, the enantioselective asymmetric hydrogenation of prochiral C=C double bonds is possible (Scheme 1.8). 

Substitution of the coordinated aqua trans to the nitrido (N3 ) ligand in complexes of the form [ReN(H20)(CN)4]2 with CN ligands in the equatorial cis plane, denoted by ReNCfROjCN. 

The Cp ligand in complex [61] can be substituted by stabilized carbene units to give 62 in Scheme 19,63 which is square planar. 

Normally, hydride can be readily a-abstracted from alkyl complexes when the metal is in a low oxidation state and bound to good n-accepting ligands. In complexes with p-hydrogen, P-elimination might become the main reaction, specially when highly substituted alkenes are formed (Figure 3.10). 

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