Ligand properties higher

In general, these distributions all share the intuitive property that the vast majority of ligands have low to moderate affinity, and fewer and fewer ligands have higher affinity. Though the Sips, log-normal and RAD distributions can be made visually quite similar, they will always differ in their moments and other mathematical properties. It should be stressed that these distributions are for affinities. For other properties, such as enzymatic activity, these distributions may not be relevant, though at least one model for catalytic activity is based on these distributions [19]. 

Many efforts have been made in the last decades to modify the electronic and steric properties of /3-diketones by synthesizing derivatives of 84 with R, and R substituents other than alkyl or aryl groups, to prepare polyfunctional coordinating ligands with higher complexity and functionality. The examples reported in this section have been limited to cases which have been studied from the redox viewpoint and, in particular, /3-diketonates derived from 113-116. 

Model complexes with simple thiolate ligands have higher symmetry than the metalloprotein sites as, in the latter case, the protein environment about each iron atom is different. Often such differences are of little significance, but in a number of cases they give rise to distinctly different properties for one (or more) of the iron atoms. These clusters are termed subsite differentiated and have been recently reviewed by Holm et al. 251). 

Charge neutralization also affects the redox properties of thioether complexes. It contributes to the marked stabilization of lower oxidation states found in all cases. Thioether complexes of metal ions in high oxidation states may approach the boundaries of the electroneutrality principle. Apart from any n-acidity of the ligands, simple electrostatic considerations suggest that poor charge neutralization by the ligands disfavors higher oxidation states. 

The superior donor properties of amino groups over alkoxy substituents causes a higher electron density at the metal centre resulting in an increased M-CO bond strength in aminocarbene complexes. Therefore, the primary decarbo-nylation step requires harsher conditions moreover, the CO insertion generating the ketene intermediate cannot compete successfully with a direct electro-cyclisation of the alkyne insertion product, as shown in Scheme 9 for the formation of indenes. Due to that experience amino(aryl)carbene complexes are prone to undergo cyclopentannulation. If, however, the donor capacity of the aminocarbene ligand is reduced by N-acylation, benzannulation becomes feasible [22]. 

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