Anionic ligands, variation

The choice of isothiocyanate as the anionic ligand is to some extent historical, but it seems to be a very appropriate ligand with an intermediate ligand field strength. The variation in ligand field strengths for a list of cyanide ligands is ... 

Further variations of the basic motif of 1 comprise carbene le in which the anionic ligands are trifluoroacetate rather than chloride (Fig. 2) [4d]. However, this specific compound shows a significant tendency to isomerize the double bonds of the substrates in addition to its metathetic activity. Moreover, water soluble catalysts have been developed that contain bulky aliphatic phosphine ligands with either a quarternary ammonium group (lf,g) or a sulfonate function (lh) [12]. They have been tested in the ring opening polymerization... 

Goldschmidt (1923) was the first to note that ionic radii vary as a function of coordination number—i.e., with the number of first neighbors surrounding the ion. The observed variation is fundamentally identical for all cations and, in the case of cations coordinated by anionic ligands, results in a progressive increase in ionic radius with increasing coordination number. As an example, figure 1.9 shows the increases in ionic radius for the various rare earth elements (and In, and Sc ) as a function of coordination number with. ... 

We see only a small variation in rate constants fora variety of anionic ligands. It... 

Further insights into the relationships between structure and catalyst activity and selectivity have been achieved through variation of the three major components the anionic ligand the non-chelating NHC substituent and the nature of the chelating substituent. 

In the cell, an area of low chloride ion concentration, equilibrium favors loss of the anionic ligands with the formation of various hydrated derivatives. It is these very reactive complexes, with their labile aquo ligands, which coordinate to DNA and lead to the observed cytotoxicity. Excepting considerations of solubility, the broad structure/activity observations of the past decade are probably a reflection of variations in only the end of this process. In particular, variations in the amine identity are likely to have only minor effects on the kinetics... 

The past decade has witnessed extensive modifications of Af-heterocyclic carbene ligands for ruthenium olefin metathesis catalysts. This includes symmetrical and unsymmetrical NHCs, 1,3- and 4,5-substitutions, introduction of heteroatoms into the backbone, NHC ring size variation, and introduction of chirality. Most of these changes were initially targeted to improve stability and activity of the catalyst, while recent approaches are mainly focused on affording well-defined stereoselectivity. However, the activity and stability of the ruthenium-based metathesis catalysts are not solely ruled by the type of neutral NHC ligand the anionic ligands, chelation mode, substrates used, and the reaction conditions naturally also influence catalytic properties. One of the main lessons learned from ruthenium olefin metathesis development is that there is no one catalyst fits all and every type of application must be studied in detail in order to discover the most efficient catalytic complex. 

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