Ligands electron donicity

An interesting aspect is the relative net electron donicities of carbene hgands. In a series of N-stabilised carbenes [ClNPr ), SIMes, Imes], Herrmann et al. showed that unsaturated NHC are better net donors than saturated NHC and sometimes can even outperform the acyclic and more basic carbene ligand ClNPr l [57,104,107] (see Figure 1.21). A similar observation was made by Dorta et al. [19]. However, the absolute differences between the net donicities of these different carbene ligands are very small and are unlikely to play a major role in catalyst performance [19]. 

It may obviously be tempting to modify the pyrazole bridge to apyridazine one [84], going from a five-membered to a six-membered heteroaromalic systan and lowering the electron donicity of the bridge unit in the process [69,70]. When Scheele et al. did just that [85,86] they found that the pyridazine ligand does not coordinate, at least not to mercury(II) [85]. 

The Grubbs catalyst requires a vacant coordination site to become active. In the active species, the NHC ligand is superior to the phosphane, simply because of the greater net electron donicity. In many other C-C and C-N coupling reactions, the picture is different. 

The ionization and oxidation (peak) potentials [53] of metal carbonyls listed in Table 3 establish their mild donicity despite their (formally) neutral oxidation state. However, partial replacement of carbonyls by stronger donor ligands (such as phosphine, sulfide, etc.) effects an incremental increase in their reducing properties [54], Such a fine-tuning of the oxidation potentials of structurally similar organometallic donors is ideal for studies of the correlation between rate constants and the electron-transfer driving force (see Section 2.5). 

Note In NMR, the influence of n-electrons on the position of the signal is significantly greater than that of cr-electrons. Therefore, the upfleld shift due to TT-backbonding can overcompensate the downfleld shift due to the cr-donicity of the ligand, even though the actual bonding contribution is in reverse order. 

Since most phosphane ligands also show some. -acceptor ability, coordination to a transition metal can result in >r-backbonding that would increase the electron density on phosphorus, and thus result in an upheld shift of the signal in the P-NMR spectrum. Depending on the magnitude of the downheld shift due to the cr-donicity, and the upheld shift due to the. -acceptor strength of the phosphane, either a net downheld or net upheld shift is observed upon coordinahon of a phosphane to a transition metal. 

In the series mer- MLfl (M=Rh, Ir), we see that A is large (in magnitude) and B is small, indicative of a much smaller influence of o donicity on AS. There is no coligand of appreciable 7r-acceptor strength present, and thus all of the. r-electron density from the metal will be taken by the phosphorus ligands. The difference between the two non-equivalent phosphorus atoms is small, both in A and in B, and we may snspect that the high oxidation state may lower the tran -influence on the [Pg.87]

A different approach in terms of donicity has been used to explain some ligand effects in redox reactions. A correlation has been found between the energy of the first d-d bands of a number of Cr complexes and their rates of electron exchange with Cr +. The tetragonal distortion of the Cr complex was evaluated from an analysis of the electronic spectrum and is manifested as a lower reorganization energy of the precursor complex. 

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