Covalency parameter

Cobalt(III) complexes of the general formula [CoL2]BF4, have been prepared for 3a [179], 9 [137], 10 [139], 11 [140], and 12 [141] and their values of Dq are 2200-2500 cm and values of the covalency parameter, B, are 550-740 cm The strongest field and most covalent bonding is provided by 11, 2-acetylpyridine C-hexamethyleneiminethioemicarbazone, which has the largest alkyl function attached at N. 

The composition of the film changes with potential and with the incorporation of both anions and cations. EXAFS data on the passive films grown on stainless steel or normal steel with Cr04 show considerable incorporation of Cr into the film with further alterations in bond lengths and covalency parameters. Indeed, as the amount of Cr incorporated increases, so does the flexibility of the structure. It is well known, by contrast, that Cl incorporation leads to poorer quality films and to enhanced rates of corrosion. 

It is interesting to note that the large difference in covalency parameters for the Mn2+—0 bond in MnO Nathans et al., 1964) vs the Mn2+—O bond in MnCC>3 (Lindgard and Marshall, 1969) agrees well with the spin transfer anticipated from Rv and R. This covalency difference for an M—0 bond in ternary oxides M Y Oz, which is dependent on the nature of the Y—0 bonds [in this case (Mn—O)—Mn for MnO and (Mn—0)-C for MnCOs], was also shown to exist for other oxides (see Table 4). 

Ten-coordination has ben claimed for M(fod)3(bipy)(H20)2, where M = Y, La, Pr, Eu or Lu, but the evidence (thermogravimetric analysis) is slender and a coordination number of eight as [M(fod)3bipy]-2H20 seems more likely, in line with the complexes M(fbd)3 phen where M = Y, La, Pr, Yb, or Lu, prepared at the same time.284 The electronic spectra were also studied and related to covalency parameters and the NMR spectra were discussed, but as might be expected, the complexes show no ability to act as shift reagents as they are already too crowded. 

It was later shown by Laurence and coworkers that there are significant systematic differences between P values of solvents obtained with indicators with an oxygen donor atom and those with a nitrogen donor atom (Nicolet and Laurence 1986). These authors recommended the use of a single indicator, preferably 4-nitrophenol relative to 4-nitroanisole or else 4-nitroaniline relative to 4-nitro-N,N-di/rae%>/aniline (rather than 4-nitro-N,N-die// y/aniline used by Kamlet and Taft 1976), to establish a basicity scale. The main point of difference is with respect to solvents that do not have an oxygen donor atom, such as amines, pyridines, and sulfides. In order to save the P scale, Kamlet and Taft proposed a family-dependent covalency parameter, equal to -0.20 for P=0 bases, 0.00 for C=0, S=0, and N=0 bases, 0.20 for -O-bases, 0.60 for pyridines, and 1.00 for amines, for use in linear free energy relationships (Kamlet etal. 1985). 

Spectral data for miscellaneous complexes are available in the references cited in Table 8.31. The covalency parameters of some complexes are given in Table 8.50 (see Appendix). 

Nephelauxetic parameter (ft) and SINHA s covalency parameter of (6) for the HMPA-complexes of the lanthanides, and a comparison with other complexes containing P -> O donor ligands. TBP = tributylphosphate, OMPA = octamethylpyrophosphoramide, HMPA = hexamethylpyhosphoramide. To emphasize the weak nephelauxetic effect produced by the P-0 donor ligands, some of the fi and S values are expressed with three or four places after the decimal without rounding off. Ref. [290]. 

Values of average nephelauxetic ratio (/3), average covalency parameter (8) and average bonding parameter (h1/2) of lanthanide 4-nicotinyltrifluoroacetonates. Ref. [291],... 

In addition to the p parameter of solvent Lewis basicity, the coordinate covalency parameter has been found to be useful in correlating certain types of so-called family-dependent solute basicity properties [226, 267]. Family-independent (FI) basicity properties are defined as those which have a linear relationship with p when all solute bases are considered together. Family-dependent (ED) basicity properties are those which exhibit a linear relationship with p only when different families of solutes having similar HBA sites are considered separately. Thus, ED properties can be correlated to FI properties if an empirical coordinate covalency parameter is used in correlation equa-... 

A relation of the type (2.23) was first used by Owen et al. (40) to rationalize the experimental covalency parameters of divalent metal ions (see below). 

Table 2. Relation of /q to covalency parameters for an octahedral complex )...

The scattering length of the calibrant must of course be accurately known, as must those of the sample if an internal calibration is being made. This was well discussed by Hutchings and Guggenheim 82) in estimating the errors associated with the covalency parameter for Ni2+—F determined in KNiFs. 

From Table 7 we see that there is no significant difference in covalency parameters for Mn2+ — F and Mn + — O -. This agrees with the neutron diffraction data for Ni2+ (Section 4.1). Although /<, is much less than for Ni2+, the total ligand-to-metal charge transfer by a and n bonding from Eq. (2.23) is very similar for the two ions ( 0.2e). 

Electron Transfer. Neta and coworkers have worked extensively with halogen-substituted methyl peroxyl radicals (X H COO , where X = Cl, Br or F) in aqueous and non-aqueous media, using combinations of solvents in different ratios to change the polarity of the mixture. They describe the mechanism for the reaction of the water-soluble antioxidant Trolox with their peroxyl radicals as H-mediated electron transfer , having determined that the rate of the reaction increases with an increase in solvent polarity. They examined solvent polarity in terms of the dielectric constant of the solvent, e, and solvent basicity, reported as either the coordinate covalency parameter, f, which is a measure of solvent proton-transfer basicity, or the value, which is a measure of solvent hydrogen bond basicity . 

The d—d transition energies can then be written down in terms of the parameters a. n ( ), r ( ), ag and a4. There is now a large body of experimental data on which to base empirical values of the covalence parameters. We shall now give tabulations of these parameters as a function of the intemuclear distance for the most important ligand atoms encountered in copper(II) compounds. From these tables it should be possible to predict the d—d spectrum of any copper(II) chromophore of known or assumed geometry. 

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