Portugaliae
Electrochimica Acta

The competition between the anodic oxidation of the illuminated n-GaAs electrode and that of tetramethyl-p-phenylenediamine (TKPD) was studied by rotating ring-disk voltammetry as a function of the photo-current density, the TMPD concentration, the pH and the solvent composition, i.e. the water to acetonitrile ratio. From the results, it is concluded that decreasing the water content of the water-acetonitrile mixture influences the kinetics in the sense of improving the stability of the phctoelectrode, and that the subsequent electrochemical decomposition steps may involve either a free hole or a surface intermediate as the mobile surface species, depending on the composition of the electrolyte. These medium effects lead to a better insight into the structure of the decomposition intermediates of GaAs and into their chemical interactions with the solvent.

The over-all formation constants of the mixed ligand complexes of 1-methylimidazole (l-Melm) and oxalate (Oxal) with cadmium (II) have been studied polarographically at constant ionic strength, p = 2.0, (NaN03) and pH 8 at 25 + 0.1°C. The reduction of the complexes at a dropping mercury electrode,DME, is reversible and diffusion controlled. The constants for three mixed complexes of Cd(II) are log = 4.0 for Cd(l-Melm)(Oxal), log p12 = 4.4 for Cd(l-Melm) (Oxal)2 and log p2± = 5.9 for Cd(l-Melm)2 (Oxal). The tendency of a ligand to add to a complex and to substitute another ligand was compared.

In the present paper we present the results of a study of the complexes of 1,4,8,12-tetra--azacyclopentadecane-N,N',N';N'''-tetracetic acid (cPENTA) which was synthesized in our laboratory. The stability constants of the complexes of Ca, Sr, Ba, Co, Ni, Cu and Zn and the thermodynamic functions for the formation of the complexes of Ni , Cu and Zn have been determined. The sequence of protonation of the ligand is discussed on the basis of the results of a NMR study and the structure of the complexes of nickel and cobalt is analysed in terms of their respective electronic spectra and of the calculated fractions of the ionic and covalent contributions of the enthalpy changes on formation of these complexes.

Reaction of [MoS^][PPh^]^ with methyl isocyanide (CNMe) affords species formulated as [Mo0S,(SCNMe)0][PPh.] or [Mo„S,(CNMe)(CNH)(SCN)][PPh.] in which the isocyanide or the derived ligands undergo cathodic CN bond cleavage to give ammonia, methane and ethylene, in processes involving sulphur as a reductant of the substrate.

The behaviour of ions in organic and mixed solvents is illustrated by a number of examples. Systematic study of the effect of solvent on potentials of cells with ion-selective electrodes led to the development by Kakabadse and co-workers of a new technique in direct potentiometry, viz. the ion isoconcentration technique (IICT). In IICT, concentration of solvent is varied and that of indicator ion is kept constant. Many simple mixtures show under ion isoconcentration conditions a systematic change of cell potential with variation in the concentration of organic solvent, providing a graphical method for the rapid determination of, (a) individual organic solvents in simple mixtures, and (b) residual water in organic solvents. A laboratory prototype monitor for the continuous determination of ethanol in cider and sherry, using the fluoride isoconcentration technique (FICT), is described.

An electrochemical study on the mesoionic heterocycle mentioned above is carried out, using dc polarography, cyclic voltammetry and controlled potential electrolysis . A reduction mechanism with a consumption of four electrons in each polarographic wave is proposed. On the other hand, we detect a kinetic response of CE type caused by an equilibrium between hydrolysis products ot the heterocycle.

The polarographic reduction of Zn(II> and Co(II) complexes with paludrine has been studied in moderated acid media. The Zn(II) 2:1 complex shows two well defined waves. The Co(II) 2:1 complex shows only one wave. Arrhenius activation energies, obtained from diffusion currents of np, dp, aci , aca, KT and K-L- polarograms, show that all cathodic and anodic processes are controlled by diffusion. The mechanism of these processes is proposed.

Cyclic voltammetric studies were performed for six redox couples at a Pt electrode, in the absence and in the presence of Bi, Pb and Cd adatoms. Standard rate constants were evaluated for all the systems in the absence and in the presence of adatoms. Whenever a catalytic effect was obtained the effect of coverage was studied in a quantitative way.It was found that the rate constant increases initially rapidly and then passes through a maximum or reaches a constant value, at coverages of the order of a monolayer. For all the systems the influence of the adatoms decreases in the order Bi > Pb > Cd.

Within our broad interest on the study of the redox and electronic (1) properties of coordination compounds by electrochemical methods, we embarked upon the investigation of the redox behaviour of series of ferrocene derivatives, FcX [where Fc stands for FeCn^-Cj-H) (n5-C H)]f with different types of (2) substituents (X); the latter may ligate an aromatic ring through, for instance, a sp or an unsaturated sp hybridized C atom, and some homo- and hetero-annular disubstituted ferrocenes have also been investigated. However, in this preliminary communication, we wish to report the results we have obtained with a type of substituents we are particularly interested from a coordination point of view: that with the isocyan functional group. In the ferrocene derivatives under study, CNR [J_; R=Fc (a), CH2FC (b)or CH(Men)Fc (c) where Men=(lR,2S,5R)-2-isopropyl-5-methylcyclohexyl], the isocyan group is prone to coordination to a metal centre and the derived ferrocenyl--isocyanide complexes [Cr(CO)^(CNR)] (2) can be prepared by reaction of the former with [Cr(CO) (thf)].