Ligand donor strength

This shows that the activation energy for elimination can be reduced by either increasing the electron density on the a-carbon (by ligands of greater donor strength) or decreasing the electron density on the jS-carbon. 

Replacement of the phosphorous ligand with an NHC is a logical next step toward stabilising the D-type intermediate due to the o-donor strength of the NHC. Thus, choosing the correct NHC should allow for high selectivities without excess ligand. 

Since the beginning of the 1980s, two different approaches to quantify the H-bond contribution to properties at the 2D and 3D levels developed independently. The carefully parameterized methodology of HYBOT allows one to take into account the influence of substituents on H-bond acceptor and donor strengths. Modern procedures based on X-ray data of ligand-macro molecular complexes consider the... 

In addition to the variation in electronic configuration, the geometric details of the coordination sphere and the properties of iron-ligand bonds (different a- or 71-donor strength) also influence the isomer shift as observed for a series of compounds ... 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

It is well-established that the molecular and electronic structures of metal complexes of azamacrocycles are greatly affected upon N-alkylation (197). This is mainly due to two factors (a) the decrease of the ligand field strength and (b) the increase in the steric requirements upon going from a secondary to a tertiary amine donor function (251). To examine whether the properties of the dinuclear amine-thiophenolate complexes are affected by the N-alkyl substituents, analogous complexes of the... 

In order to compare donor strengths of different donor ligands, Su and Collette found it convenient simply to use the pKa values (23) of the onium salts of various donors. This is based on the assumption that the complexation of Rh with a donor is analogous to an acid-base interaction ... 

---