Additive 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... 

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. 

A distinct electrochemical ligand parameter, El, has been proposed recently [9] and used to predict the redox potential of complexes by assuming an additive contribution of all their ligands. The empirically derived relationship can be given by eq. (6), expressed in volt vs. N.H.E., where and 1 depend upon the metal and redox couple, the spin state and... 

Linear correlations between the Tc reduction potentials and the summation of the electrochemical ligand additive parameter ( E El ) [ 8 ] have been recognized [ 1 ] for the Tc(II,I) couple — apart from the higher oxidation state Tc(III,II) and Tc(IV,ni) redox pairs - with observed least-squares slope and intercept values [ 1, 9 ] of =1.40 and I = -2.08 V. Our experimental results [Ep/2 vs. NHE ( Table )], although considerably away from this correlation ( e,., with a difference of -1-0.34 or -0.24 V between the expected and the observed redox potential for the CO or CNCgH i complex, respectively ), are still within the scatter of the data used ( over a wider EELrange ) to determine [ 1 ] such a relationship. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

Most polypyridine complexes of second- and third-row transition metals also display a predominantly metal-localized oxidation at positive potentials which are chemically either reversible or partly reversible. Further one-electron oxidations often occur at more positive potentials in liquid SO2 [57]. The first oxidation potential depends on the metal atom (for example, Ru > Os), the ancillary hgands in [M(W)(X)(Y)(Z)(N,N)j or [M(X)(Y)(N,N)2] and, also, on the structure of the polypyridine ligand. Empirically, oxidation potentials can be calculated using additive Lever electrochemical parameters which quantify the influence of the metal atom and individual hgands on metal-centered redox couples [9, 157, 220]. 

Chemical composition of the electrolyte is a particularly important parameter in PEC systems based on complex electrolytes snch as polysnlphide or ferro/ ferricyanide. In the latter redox conple, as shown in Fig. 10.8, replacement of one of the hexacyano ligands strongly changes the photoelectrochemical response of illnminated n-CdSe dne to a combination of electrochemical and spectroscopic effects (Licht, 1995), and addition of the KCN to the electrolyte can increase n-CdSe and n-CdTe photovoltages by 200 mV (Licht and Peramnnage, 1990). 

In common with Enemark—Feltham, the new notation makes no attempt to define the formal charges on the nitrosyl ligand and the formal metal oxidation state but focuses attention on the geometry of the nitrosyl, the metal s coordination number and the total electron count. As De La Cruz and Sheppard have recently pointed out [26] in their extensive analysis of the vihratiOTial data for nitrosyl complexes, the great majority of them conform to 18- and 16-electron rules, and therefore, this parameter establishes whether the molecirle has a closed shell. The total electron count has important chemical implications since it indicates whether the compound is likely to undergo electrochemical conversion or nucleophilic addition in order to achieve an 18-electron configuration. 

Extensive data are now available to quantify ligand additivity effects on E values. In a detailed study by Lever, " ligand electrochemical parameters for over 200 ligands are presented and the model proposed has been tested with a wide range of coordination complexes (see Section 2.15.2.4). In the more sophisticated models, the E value is described in terms of the sum of factors involving the ligand electrochemical parameter and the metal. As expected, Ef depends on the... 

When a single EAC predominates in the system, the determination of kinetic parameters should present few problems, and a quantitative description of electrochemical kinetics becomes comparatively simple. This book contains a number of examples when simulated and experimental data coincide well. Among these are prewave-containing voltammograms that are typical of ligand-deficient systems. In addition, theoretically predicted anomalous high slopes ofvoltammet-ric characteristics were observed experimentally for reduction of Sn(II)-citrate complexes. 

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