Correlations donor number

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

The best-known solvent parameters are the donor number [21] and acceptor number [22] proposed by Gutmann and coworkers. The donor number (DN) for a donor solvent D is defined as the positive value of the enthalpy difference AH (kcalmol ) for the reaction of D with an acceptor-halide SbCls (D + SbCls D SbCls) in an inert medium such as 1,2-dichloroethane. DN is a fair measure for the donor properties of a solvent. The correlations of DN with the solvation energies are known to be good particularly for solvation of cations. A typical example [19] is shown in Fig. 3. 

Generally the magnitudes of solvent exchange rate constants increase in the sequence NH3 > H20 > DMF > MeCN > MeOH, which is largely independent of the nature of the metal ion (108,109). This sequence cannot be readily identified with specific characteristics such as dielectric constant, donor number, electric dipole moment, or stereochemistry, and it appears to reflect the overall solvent characteristic. There may be a correlation between the AH for solvent exchange on [M(solvent)6]2+ and the heat of dissociation of solvent from this species (110). 

There is no correlation between Z values and donor numbers, as they measure different properties. Other methods of estimating solvent polarities have been discussed by Kosower, who has tabulated values for many of them 147F... 

The metal-solvent interaction is expected to depend on the donicity of the solvent the higher the donor number of the solvent the stronger the solvent-metal interaction should be. Hence, a correlation between the contact potential difference A>// (a = 0) and the donor number of the solvent should be observed. However, this correlation for the Hg electrode is rather poor, with the most deviant point having been found for water, that is, for the case of the strongest dipole-dipole interaction in the bulk. The correlation is better when acceptor numbers of solvents are taken into account. ... 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

FIGURE 1. Dependence of the log of conductivity of 0.1 M solutions of le measured at 20 °C on the solvent donor number DN. The correlation coefficient is given... 

Owing to the presence of the amine and cyanide ligands, known to give rise to specific donor-acceptor interaction with solvents [126-130], an interesting solvatochromic behavior is observed for these species. For complex 1 the spectral changes are dominated by amine interactions with the solvents as shown by the linear correlation of the solvent donor number [131] with the IT band maxima and with the half-wave potential of the ruthenium amine moiety. 

These donor numbers provide an interesting comparison of the relative donor abilities of the various solvents (Table 10.4). ranging from the practically nonpolar 1,2-dichloroethane to the highly polar hexamethylphosphoramide. ((CHdjNIjPO. Note, however, that there is no exact correlation between donor number and permittivity. Some solvents with relatively high permittivities such as mtromethane and propylene carbonate (e/e = 38.6 and 65.1) may be very poor donors (DN — 2.7 and 15.1). Conversely, the best donors do not always have high permittivities pyridine (DN -33.1. e/e0 = 12.3) and diethyl ether (DN = 19.2. e/% = 4.3). This should serve lo remind us that solubility is not merely an electrostatic interaction but that solvation also involves the ability to form covalent donor bonds. Note that pyridine may be considered to be a relatively soft base (Chapter 9). Gulmann has extended the concept... 

Cu(acac)(tmen)]BPh4, is known to provide a good correlation between the donor number (DN) of the solvent and the /,max corresponding to the lowest energy of d-d transition [84], In spite of the small number of experiments, there is a certain relation between anion species and T.max, as shown in Table 3.10. 

Table 8 compares oxidation potentials of selected solvents and their donor numbers [100], This table demonstrates that there is a correlation between higher donor numbers and lower stability limits for oxidation. A similar correlation for a larger variety of solvents presented by Zyat kova et al. [104] for LiAsF6 solutions is shown in Table 9. 

Table 8 Correlation between Donor Numbers and Oxidation Potentials for Selected Solvents... 

In asymmetric complexes of the type [(bpy)2RuCl(pi-pyz)Ru-(NH3)4L]4+, studies (94) revealed that there is a solvent donor-number (DN)-dependent contribution to the Frank-Condon barrier of approximately 0.006 eV/DN, which completely overwhelms the dielectric-continuum-theory-derived (l/Dop-l/Ds) solvent dependence typically observed in symmetrical dimers. In this case, variations in MMCT Eop with solvent give linear correlations when plotted against solvent dependent AEm, the difference in potential between the two ruthenium(III/II) couples, as shown in Fig. 10. The microscopic origin of this solvent effect was described by Curtis, Sullivan, and Meyer (122) in their study of solvatochromism in the charge transfer transitions of mononuclear Ru(II) and Ru(III) ammine complexes. The dependence... 

The donor number has proven very useful in coordination chemistry, since it can be correlated with other physical observables for such reactions, e.g. thermodynamic... 

The solvent-induced change in rate is, however, much larger than expected from the relatively small difference in polarity between nitromethane and hexamethylphos-phoric triamide. This, together with the correlation between rate decrease and increase in the solvent donor number DN cf. Table 2-3 in Section 2.2.6), suggests that specific solvation and stabilization of the diazonium ion by EPD solvents play a dominant role in the reaction (5-27). Very likely, formation of an EPD/EPA complex between the reactants in a rapid preequilibrium step precedes the rate-controlHng first step [504, 792],... 

Donor numbers are considered as semiquantitative measures of solute/EPD solvent interactions, and are particularly useful in the prediction of other EPD/EPA interactions in coordination chemistry. Numerous examples of the application of donor numbers have been given by Gutmann [26, 27, 30] cf. also [113, 133]. It has been shown that donor number correlations are parallel with correlations based on the highest occupied molecular orbital (HOMO) eigenvalues of EPD solvent molecules [139], For non-HBD solvents, a fair correlation has been obtained between their donor numbers and their gas-phase proton affinities FA, indicating that the DN values do indeed reflect the intrinsic molecular properties of EPD solvents [140]. 

This 5phOH scale was further extended by Makitra et al. [242]. It has been shown that the spectroscopically determined 5phOH values correlate well with Gutmann s calori-metrically measured donor numbers DN [243] cf. Eq. (7-10). 

A series of silanol and silylamine chemical shifts were obtained in various solvents. (83) The silanols are found to be highly dependent (>5ppm shifts) upon solvent basicity with the more basic solvents causing low frequency shifts. This shielding effect is found to give an excellent linear correlation with Gutmann s donor number (DM) (130) which is a measure of the electron pair donor ability of the solvent. Figure 23 shows the correlation for five of the compounds examined. It... 

Various criteria have been followed in an attempt to establi quantitative scales of acidity and basicity. In order to account for solvation and ionic dissociation phenomena Gutmarm introduced a parameter called donor number, DN, which correlates the behaviour of a donor sdvent towards a given solute with respect to the coordinating ability of a reference solvent towards the same sdute. The basicity of a solvent can be related to the enthalpy of its reaction with a reference acid. Gutmann s DN scale is built on the equation... 

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