Electrochemical ligand parameter

Properties which are relevant for bond stabilities are closely related to orbital energies, and the latter can be measured directly by electrochemical procedures. Hence the observed relationship between the electrochemical ligand parameter E (L) and bond stability -even including hydrolytic stability rather than bond energies in the gas(eous) phase - might have been anticipated. This correlation is based upon the log-linear regression equation... 

Change of electrochemical ligand parameters (Lever 1990) and of redox potentials... 

Fig. 2.2 Relationship between electrochemical ligand parameters of some bidentate ligands and their first complex formation constants with M = Al, Cu, Mn(ll) and Co(ll). Note the different sign of slope that is, with complexes get the more stable the lower their E (L) values are. Note that the sign of the slope (corresponding to x) differs (positive for three metal ions, negative with Mn +). This is presumably due to the different number of d-electrons in d -Mn(ll) complexes, unlike Cu(II) or Co(ll), spreading of d-orbital energy levels wiU not cause decrease of redox potential but rather the opposite. With x expressing the...

Equation 2.4 combines two parameters which are typical for both the metal ion and the way the ligand is bound the parameters are called (ligand) sensitivity x and intrinsic bond stability c. c is an acronym of con-stans, alluding to the fact that this part of bonding does not depend on the electrochemical ligand parameter while the sensitivity x does. X thus refers to the iden-... 

How Does the Electrochemical Ligand Parameter Influence Real Versus Possible Hapticity of Some Polydentate Ligand ... 

Yet there is no relationship between hardness of some donor and its electrochemical ligand parameter, as can be seen from the sequence of Ej (L) according to... 

Table 2.5 c and x values and relative stabilities of bi- vs tetradentate binding of some ligand. The critical values correspond to some sum of ligands, owing to the definition of the electrochemical ligand parameter... 

Here, critical electrochemical ligand parameters are mnch closer to each other and to the realistic range of EL(L) than for the 2/4-eqnilibrinm discussed before in addition, the complex formation constants... 

Equation 2.4 can be rearranged in order to calculate an electrochemical ligand parameter from complex stability data provided both hapticity of ligand and c and x of the central ion are known. The results are compiled in Table 2.2, rearranging Eq. 2.4 in modified form (2.7) as to obtain E (L) ... 

Table 2.7 Data for effective electrochemical ligand parameters of several different plant species. Values were calculated using...

Implication for biomonitoring corrections by use of electrochemical ligand parameters and BCF-delined element clusters... 

Highly negative values of k in addition imply the possibility to retain or even enrich elements which will form but rather labile complexes given the effective electrochemical ligand parameter of the plant species and (Eq. 2.11). These include Sr, Ba or Mn and the REEs (except of Sm, Tb) if E (L) j is close to zero in the latter case, all of which are known to be hyperaccumulated in some plants, e.g. Ba and Mn in Brazil nuts (Emsley 2001) and - among our test set of plant species - Mn gets substantially enriched in blueberries (both leaves (to which data reported here (Markert 1996) pertain) and fruits), k thus is a kind of measure for amplification of differences in the sequence of transport within some plant, from sequestration in/ by root exudates to deposition in the tips of leaves. 

In both kinds of Vaccinium there is potential for thorough amplihcation of small differences in complex stabilities within the plant and thus for hyperaccumulation this mainly refers to Vaccinium myrtillus as its effective electrochemical ligand parameter is fairly low (-0.25 V). Grasses and trees (both deciduous ones and coniferes) vary around k 0, except for Deschampsia flexuosa. 

The Electrochemical Ligand Parameter, Metal Affinities and Chemical Ecology... 

The corresponding equilibrium state of distributions of various metal ions usually differs from those of the soil substrate, hence entails some fractionation during transport as described before. So, fractionation can be compared to that effected by some single ligand, and Eq. 2.4 and its inversion are then used to define some effective electrochemical ligand parameter which describes the capability of some plant organ to fractionate among metals. 

Therefore no longer qualitative statements are cast into a numerical framework but easily measurable values are linked to obtain information on complex stability or bond energies. Thus, by linear regression analysis, two new parameters are obtained which -once they are known for sufficiently many different metal ions, both essential and non-essential ones - in turn can be linked to this biochemical property of essentiality. Electrochemical ligand parameters for different complexes of the same metal ion are correlated... 

Fig. 2.15 Element fractionations and effective electrochemical ligand parameters for photosynthetic organs of five plant species frequently occurring in forest understorey regions (fmm left to right) Vaccinium myrtillus (blueberry), Lolium perenne (ryegrass), Deschampsia flexuosa, Molinia caemlea and Vaccinium vitis-idaea (red whortleberry). For other domestic plants, E (L) values may be higher or lower than the average of the above values, e.g. -0.19 V for each Lolium perenne and Betula alba. Very low effective electrochemical hgand parameters (oak, dandehon, plants in tropical...

Given hapticity (denticity) and electrochemical ligand parameter are identical, a certain ligand - say, the anion of diacetyldioxime - would exert the same discriminating effect on some set of metal ions as the kind of biomass, except for some amplification term (see below). This matches the way of metal transport and accumulation in and among different plant organs as described by Clemens et al. (2002), fractionation being due to different values of sensitivity and intrinsic bond stability. 

When analyzing corresponding reaction networks in more detail, as will be done for Mg afterwards (Section 2.2.12), the electrochemical ligand parameter and its changes must be considered once again to understand transformations. For now, this is not yet feasible for fluxes of a multitude of essential metals just because informations on the involved chelators are missing this precludes the kind of analysis done for Mg before, notwithstanding issues like redox speciation. As, moreover there is neither... 

Table 2.16 Effective electrochemical ligand parameters, kinds of metal ions on which the calculation is based (there are no omissions for reasons of mismatch ) and biochemical effects in Daphnia magna (Franzle and Markert 2006a). For a denticity of 2, consistent values were obtained in aU cases. The rightmost column is for illustration only and not meant to suggest that these ligands are directly involved in the above (column 1) effects of metal administration. The values were derived from regression analysis of toxicological activities or BCF values (Weltje 2003) after rearrangement of Eq. 2.1 for the electrochemical ligand parameter...

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