Redox potentials, stabilization complex formation

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus, electron transfer by the Fe(II), Fe(III) system can occur at pH = 7 over the entire range of the stability of water Eh (-0.5 V to 1.1 V). 

Complete sequences of ca. 50 different plant-type ferredoxins(Fd) are known. The invariant sequences nearest to the 2-Fe core are confirmed to be Pro-Tyr-Ser-Cys-Arg-Ala-Gly-Ala-Cys-Ser-Thr-Cys-Ala-Gly and Leu-Thr-Cys-Val. 2Fe-2S complexes of oligopeptides with the Cys-X-Y-Cys sequence have been synthesized by ligand exchange reactions (7,23). We have examined the redox potentials of these model complexes, and the results are shown in Table I. The reversibility improved remarkably and the potential approached the value of the native proteins as the sequence more closely simulated that of the proteins. It is conjectured that hydrogen bonds from the peptide N-H s to thiolate and/or sulfide groups increase the stability of the reduced cluster. It is likely that peptide sequences like those found in the proteins favor the formation of such hydrogen bonds. 

Some of the complexes were found to exhibit redox potentials that shifted with temperature (230-300 K). The complexes with Cys-Gly-Ala and Cys-Gly-Ala-Cys sequences were found to be particularly susceptible to temperature variations in CH2CI2 (13,14) with positive shifts at (ca. 0.10 V). These shifts were rationalized by the formation of NH—S hydrogen-bonded conformers (see Fig. 5), which are expected to stabilize the reduced forms of the complexes Such a hydrogen-bonded structure has been deduced from the X-ray analysis of several bacterial Fds (26). 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

Due to a larger cavity size, the redox potentials of the compounds with capten ligand are higher than those of Ne-sarcophaginates. The X-ray data for [CoiiTazacapten)] + and [Co (azacapten)]2+ cations (see Section 3.1) reveal two factors that govern the E value. These are strain enhancement and lowering of the stability of the cobalt(III) clathrochelate and increases in the stability of the cobalt(II) clathrochelate. The latter is stipulated by the formation of the low-spin cob alt (II) complex. 

Figure 8.26. Representative Fe(II)/Fe(III) redox couples at pH = 7. (phen = phen-anthroline sal = salicylate porph = porphyrin = valid for [HCO = 10 M.) Complex formation with Fe(II) and Fe(III) both on solid and solute phases has a dramatic effect on the redox potentials thus electron transfer by the Fe(II),Fe(III) system can occur at pH = 7 over the entire range of the stability of water (-0.5 to 1.1 V). (= Fe 0)2 Fe refers to Fe adsorbed inner-spherically to a surface of a hydrous ferric oxide. The range of redox potentials for heme derivatives given on the right illustrates the possibilities involved in bioinorganic systems.

Complex Early elements show hard complexing behavior later ones have an increasing affinity for formation ligands such as NH3. Complexing can alter redox potentials, and stabilizes some states such as... 

---