Ligands donicity

On the other hand, Table 9 contains several data which cannot be interpreted by the relative donicities of donor solvent and competitive ligand the donor strength of the iodide ion is comparable to that of acetonitrile and the formation of tetraiodocobaltate in solvents of high donicity, such as DMA, DMSO and HMPA may not be anticipated. The formation of this species is however, complete with excess iodide ions and is due to the small values of the free enthalpies of the gas phase reactions ... 

Lack of thermodynamic data prevents our making full use of the relationship between standard redox potential and free enthalpy of solvation. For many simple redox systems the standard redox potential has been found to be related to the donicities of the respective ligands. This question has been extensively discussed in a previous paper (3). 

According to the functional approach, the first stabilization rule is considered to result from the dispersion of positive charges from the coordination center over the whole complex and hence from a decrease in the net positive charge of the redox-active species, which corresponds in extent to the EPD properties of the ligands. The relationship between the standard redox potential and the EPD properties (donicities) of the ligands has been clearly established. 

In pentacyano complexes of cobalt(III) the net charge at the coordination center is considerably decreased by the strong coordinate bonds of the five cyano groups. Thus the EPA properties of cobalt(III) are considerably lower in this complex unit, but further stabilization may still be effected by a sixth EPD ligand of high donicity like ammonia. By the reaction... 

On the other hand, the axial EPD molecules only slightly affect the cobalt(II)-cobalt(I)redox potentials, since the interactions of both cobalt(II) and cobalt (I) with strongly basic Hgands are about equally weak. On the other hand, the Co(II)-Co(I) redox potentid is strongly influenced by the donicity of the equatorial ligand. A linear free relationship was found between the half-wave potential E1/2 and the nucleo-philicity of the Co (I) species ... 

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). 

The marked increase in lability of the former compound relative to the zwitter-ionic compound can be attributed to the cr-donicity of the second chelating dime-thylamino group and/or the increased chelation effect of the terdentate ligand. 

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]

We can verify that statement by looking at a pair of iridium complexes, trans-[Ir(CO)L2Cl] with an Ir(I)-center, and mer- h CO)L C with an Ir(III)-center. The two complexes differ in that the latter is derived by oxidizing the square planar Ir(I)-complex adding chloride ligands to the axial positions (see Fig. 7.1). Whereas the A value is virtually unaffected by this oxidation (-0.357 and -0.377, respectively), the B value decreases from 29.11 to 0.70 upon going from Ir(I) to Ir(III). In other words, the influence of oxidation state of the central metal is increased. 

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