Nephelauxetic series

An explanation for the nephelauxetic series came readily to hand. Lets see how successfully we can provide one for the spectrochemical series. First, note that Aoct values increase with decreasing size of the donor halides ... 

Both phenomena attest to the covalency of the chemical bonding in these species. Incidentally, they also highlight the different characters and implications of the spectrochemical and nephelauxetic series. Within either lanthanoid- or (higher oxidation state) J-block species, the ligand orbitals overlap with the metal s functions... 

J) -CHj, 318 (74) NH3, 345 (II) HjO, 380 ( 75). This shows that both organo-ligands exert a very strong ligand field comparable with that of CN and establishes their position in the spectrochemical series. It is a pity that the second d-d transitions ( T2g-(- A,g), which would establish the position of these ligands in the nephelauxetic series, have not yet been reported. Two bands have, however, been reported for the binuclear complex [(NC)5Co(CFjCFj)Co(CN)5] - at 320 and 276 nm (I2I). 

These have been calculated from Caro s spectroscopic analyses [35]. The ligands come from opposite ends of the nephelauxetic series, so for a lanthanide reaction, A rep(irteg) should be relatively large. Even so, although it proves to be the largest contributor to the overall change, AEqs and AEso are significant Quantitative analyses of claimed examples of the tetrad effect must take such terms into account... 

Notice that, with the current data, one cannot draw immediate parallels with the spectrochemical series (nor with the nephelauxetic series). At any rate, these comparisons are not trivial depending on the symmetry, CF parameters of orders 4 and 6 are more or less important compared with those of order 2. Thus, the ratio charge/distance cannot generally quantify the strength of the LF exerted by a kind of ligand, or at least not in a way that is totally independent of the geometry of the complex. 

In terms of a ligand s polarizability or covalent donor ability, the nephelauxetic series is of use for comparative purposes (318). A series 0-Me2S0 0-(CH2)4S0 > H20 > C5HsNO has been presented (163), but other data (101) suggest that 0-Me2SO be placed between the N-SCN- and CN- ions. A generally accepted series compiled from many results (319) is I- > Br > CN- Cl- > IV-NCS- > [oxalate]2- en > NH3 > urea > H20 > F-. As this series is approximately independent of the metal ion in the complex, some discrepancy is apparent. 

Reports dealing with the electronic spectra of R-dtp complexes are listed in Table 9. Several papers 120,122,123,252,284) have been concerned with the place of R-dtp in the spectrochemical and nephelauxetic series, particularly with reference to other sulfur ligands. The azide ion has been classified 340) in the spectrochemical series between S-bonded thiocyanate and ethyl-dtp and in the nephelauxetic series between bromide and ethyl-dtp. The spectrochemical order of the ligand -S2PX2 according to X has been found to be... 

The V=0 stretching frequency is an important characteristic of oxovanadium(IV) complexes, generally observed at 985 50 cm-1.353,355 It is quite sensitive to the nature of the ligands. Donors that increase the electron density of the metal reduce its acceptor properties towards O, the V—O multiple bond character and the stretching frequency. For complexes VOL , v(V=0) falls in the order L = H20 > NCS- > CN- > DMSO F this series bears no obvious relation to the spectrochemical and nephelauxetic series.353... 

Indirect evidence that electrons are shared between the ligands and the central metal ion comes from the nephelauxetic effect. It is found that the electron-electron repulsion in complexes is somewhat less than that in the free ion. From data derived. from electronic spectra of complexes, separate nephelauxetic series may be set up for... 

A nephelauxetic series - analogous to the spectrochemical series - can be constructed. It is quite different from the spectrochemical series and is consistent with the interpretation of the effect given in the previous... 

Figure 1-8. The nephelauxetic series arranges ligands in the order of their effect upon the Racah B parameter. This equates to the degree of covalency within the metal-ligand bonding. Notice that the order is dramatically different from that of the spectrochemical series in Figure 1-7.

There is a close connection between the usual chemical feeling of how complexes containing relatively reducing ligands and relatively oxidizing central ions tend to form the most covalent bonds and the sequence expressed in the nephelauxetic series. Actually, the Pauling electronegativities fall in the order... 

Such an order is difficult to rationalize in terms of electrostatic energies embodied in the simple point-charge model of crystal field theory. For example, the charged O2- anion precedes the dipolar H20 molecule in the spectrochemical series. Note that the spectrochemical series differs from the nephelauxetic series discussed in chapter 11 (eq. 11.6), which is a measure of the degree of covalent bonding. 

Such an order is intuitively one of increasing covalent bonding characteristics of the ligands based on their polarizabilities. The nephelauxetic series departs significantly from the spectrochemical series described in 2.9.2, which is based on relative values of the crystal field splitting parameter, A [see eq. (2.19)]. 

By maintaining the ligand constant a nephelauxetic series for cations may be derived. For example, from absorption spectra of metal fluoride compounds, the order of decreasing nephelauxetic ratio is... 

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