Chromium complexes electrochemistry

Table IV lists a series of octahedral (phenolato)chromium(III) precursor complexes that contain one or three oxidizable coordinated phenolato pendent arms (146, 154). These complexes display characteristic electrochemistry Each coordinated phenolato ligand can undergo a reversible one-electron oxidation. Thus complexes with one phenolato moiety exhibit in the C V one reversible electron-transfer process, whereas those having three display three closely spaced (AE1/2 250 mV) ligand-centered one-electron transfer processes, Eqs. (7) and (8).

Cyclic voltammetry and controlled-potential electrolysis are the techniques that have been used to investigate the electrochemistry of oxo-chromium and oxo-molybdenum corrolates. The data have been related to those obtained for similar porphyrin complexes. Redox potentials are reported in Table 17. 

Data on the redox potentials of germylenes, stannylenes, plumbylenes and their complexes are scarce. In fact, only the electrochemistry of dihalogermylenes, dihalostan-nylenes and their complexes with Lewis bases338 as well as with chromium, molybdenum and tungsten pentacarbonyles339 has been studied. 

The nitrogen atom in a-ferrocenylalkylamines generally shows the same reaction pattern as that in other amines alkylation and acylation do not provide synthetic problems. Due to the high stability of the a-ferrocenylalkyl carbocations, ammonium salts readily lose amine and are, therefore, important synthetic intermediates. Acylation of primary amines with esters of formic acid gives the formamides, which can be dehydrated to isocyanides by the standard POClj/diisopropylamine technique (Fig. 4-16) [92]. Chiral isocyanides are obtained from chiral amines without any racemization during the reaction sequence. The isocyanides undergo normal a-addition at the isocyanide carbon, but could not be deprotonated at the a-carbon by even strong bases. This deviation from the normal reactivity of isocyanides prompted us to study the electrochemistry of these compounds, but no abnormal redox behaviour, compared with that of other ferrocene derivatives, was detected [93]. The isocyanides form chromium pentacarbonyl complexes on treatment with Cr(CO)s(THF) (Fig. 4-16) and electrochemistry demonstrated that there is no electronic interaction between the two metal centres. 

Scheme 7-1 shows a series of ferrocenyl-benzenetricarbonylchromium(i) complexes that have been studied by electrochemistry, in which the chromium atom assumes a pseudotetrahedral geometry. 

There have been numerous publications concerning electron transfer processes with the participation of macrobicyclic complexes. Moreover, cobalt(II) and cobalt(III) complexes have received the most attention, presumably due to their availability. Several papers deal with electrochemistry of chromium, ruthenium, rhodium, manganese, nickel, iron, and copper complexes. 

Chromium(I) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2,2 -bipyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 cyanides, 703 fluorophosphine, 716 isocyanides crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 1,10-phenanthroline, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 tertiary phosphines dinitrogen, 713 tris(bipyridyl)... 

The earliest electrochemistry of chromium porphyrins involved Gr(III) derivatives with anionic axial ligands and/or coordinated nitrogeneous bases, but a number of studies have since been carried out with complexes having Cr(II), Cr(IV) or Cr(V) central metal ions. The latter series of compounds are exemplified by the oxo-Cr(IV), oxo-Gr(V), and nitrido-Cr(V) [143-147] derivatives. Cr(IV) /r-oxo dimers are also known [148,149]. 

The application of electrochemistry to the investigation of new organometallic molecules is illustrated in this contribution on two groups of recently synthesized Fischer-type aminocarbene complexes of chromium, tungsten, and iron involving about 40 derivatives. All of them represent molecules with two separated redox-active centers where the extent of their interaction (mutual influence) is given by the electron delocalization of the bridging unit, by the distance between the centers and/or by sterical effects. 

Thus, on the basis of the spectral data alone we cannot draw a conclusion concerning the preferable dissolution order of the above mentioned elements. However, the absence of absorption peaks of MoClg , NiCl4 , TiCls " complex ions indicates that these elements do not transfer to the electrolyte during the anodic dissolution of steel. This was also confirmed by the results of the chemical analysis of melt samples. From the electrochemistry point of view, the fact that electropositive nickel [30] and molybdenum [31] remain in the anode material points to the electrochemical nature of the corrosion process. The results of the spectroscopy measurements and chemical analysis were confirmed by X-ray microanalysis - the electrode surface after 2 h of anodic dissolution was slightly depleted in iron and chromium and was enriched in nickel. 

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