A.A. Vlček

Electrochemistry of Fe(CO)2(η5-C5H5)X-series. I. Detailed mechanism of reduction of [Fe(CO)2(η5-C5H5)]2 and [Fe(CO)2(η5-C5H5)]2Hg

Abstract Detailed electrochemical and spectroscopic study of the mechanisms of electrode reaction of [Fe(CO) 2 (η 5 -C 5 H 5 )] 2 , [Fe(CO) 2 (η 5 -C 5 H 5 )] 2 Hg and Fe(CO) 2 (η 5 -C 5 H 5 ) − at dropping mercury electrode has shown that the electrode reactions proper are coupled with chemical reactions between reactants and primary products of electrode reactions. The most abundant intermediate has been shown to be [Fe(CO) 2 (η 5 -C 5 H 5 )] 3 Hg − , which was characterized both at the electrode surface as well as in solutions after electrolysis. Even though some features of the polarographic behaviour of several compounds of the series Fe(CO) 2 (η 5 -C 5 H 5 )X were described [1–5], a more detailed investigation has shown the mechanism of electrode processes of the above compounds to be more complicated than described in the literature. Furthermore, the aim of our work was to follow the dependence of the electrode behaviour upon the type of X to establish the structural factors governing the course of the electrochemical processes, especially the stability of reduction products. The following groups of X were followed: 1. X = Cl. Br, I, H 2. X = GeCl 3 , SnCl 3 3. X = Si(C 6 H 5 ) 3 , Ge(C 6 H 5 ) 3 , Sn(C 6 H 5 ) 3 4. X = CN In this paper the reinvestigation and new study of [Fe(CO) 2 (η 5 -C 5 H 5 )] 2 , [Fe(CO) 2 (η 5 -C 5 H 5 )] 2 Hg and Fe(CO) 2 (η 5 -C 5 H 5 ) − is described as these compounds are essential intermediates or products of the electrode processes of most of the above mentioned compounds.

The parallelism in the mechanisms of the one-electron oxidative additions of ortho- and para-quinones to Co(CN)3−5

Abstract Ortho- as well as para-quinones were found to add oxidatively to the typical metal-centered radical, CoIII(CN)3−5, producing in aprotic media radical adducts of Co(suIII) with p-semiquinone o monodentate o-semiquinone radical ligands, respectively. The electronic structure and bonding is quite similar in the adducts of both o- and p-quinones as is reflected by EPR and UV-VIS spectra. These radical adducts are rather labile, releasing free semiquinone radicals. The adduct of monodentate o-semiquinone represents a novel structural type of o-semiquinone bonding. It undergoes a slow follow-up intramolecular substitution reaction, producing a complex with the bidentate o-semiquinone radical ligand chelated to CoIII.

Oxidative addition of quinones to planar cobalt(II) dithiolato, dithioacetylacetonato and Schiff-base complexes

Abstract The reactivity of ortho - and para -quinones with various four-coordinated planar Co(II) complexes was investigated. The o -quinones add oxidatively, producing Co(III) complexes containing chelated o -semiquinone radical-anions. No coordination of the fifth ligand in the axial position is involved in these reactions. The reaction between o -quinones and Co(II) dithiolates represents the first known example of oxidative addition to Co(II) dithiolato complexes. All observed oxidative additions are reversible; the position of the equilibrium depends strongly on the nature of the equatorial ligand. The extent of radical adduct formation decreases in the order: dithioacetylacetonate > Schiff bases > dithiolates. When redox potentials of the reacting species are changed in a way that makes simple electron transfer between Co(II) complexes and quinones thermodynamically possible, the reduction of quinones to free semiquinone radical-anions becomes competitive with the oxidative addition. In the case of p -quinones, only electron transfer is observed if the thermodynamic conditions are met. The structural factors determining quinone reactivity are briefly discussed.

Electrochemical reduction of ligated species 2,2′-bipyridine and 4,4′-diphenyl-2,2′-bipyridine

Abstract Unligated 2,2′-bipyridine and 4,4′-diphenyl-2,2′-bipyridine are reduced at low temperatures in two and three waves, respectively, with formation of the mono- and di-anion, as well as the trianion in case of Ph 2 -bpy. The corresponding M(CO) 4 bpy complexes exhibit two reversible waves at low temperature, increase in the temperature results in loss of the chemical reversibility of the second wave. M(CO) 4 Ph 2 -bpy complexes are reduced in four one-electron steps at low temperatures with formation of a series of anions ranging from the mono- to the tretra-anion. These four waves correspond to the acceptance of the electrons into two redox orbitals. The second redox orbital exhibits a rather small shift upon ligation and no dependence upon the nature of the metal. This is explained in terms of a shielding effect of the electrons already accepted.

Metal-localized protonation of Mmnt3−2 (M = Rh, Co) complexes

The reaction of electrochemically generated Mmnt3−2 (M = Rh, Co, Ni) complexes with weak protonic acids was studied using various electrochemically techniques. Complexes with M = Rh and Co react under the formation of a protonated species from which hydrogen is evolved at a dropping mercury electrode at potentials more negative than are those of the Mmnt2−2/Mmnt3−2 couple. Comparison with the behaviour of other protonated transition metal complexes led to the conclusion that the protonation site in Mmnt3−2 units is the metal atom. Protonation of Rhmnt3−3, prepared by reduction with LiAlH4, was studied spectrophotometrically in solution. [Rh(H)mnt2]2− gives a characteristic band at 464 nm. Reduction of the latter species leads to Rhmnt3−2 whereas oxidation results in Rhmnt2−2. [Rh(H)mnt2]2− decomposes slowly in solution under the formation of Rhmnt2−2 and hydrogen evolution; the stability of the corresponding Co-species is considerably smaller. Nimnt3−2 is oxidized rapidly by the weak acids used to give Nimnt2−2.

Coordinated p-benzosemiquinone radical: formation of [Co(CN)5(2,6-di-But-1,4-semiquinone)]3− by one electron oxidative additions

Abstract The product of the reaction of Co(CN 3− 5 with 2,6-di-Bu t -1,4-benzoquinone (DTBQ) in N,N′-dimethylformamide was characterized by EPR, UVVIS spectroscopy and polarography as a Co III complex with coordinated DTBQ⨪semiquinone radical, [Co(CN) 5 (DTBQ)] 3− . This compound undergoes subsequent substitution reaction releasing free DTBQ⨪ radical. The studied reaction is a one electron oxidative addition. The structure of [Co(Cn) 5 (DTBQ)] 3− and the reaction mechanism are discussed.

Electrochemical oxidation of cis-[Mo(CO)4(2,2′-bipyridine)] coupled with ligand substitution reactions in non-aqueous solvents

Abstract The electrochemical oxidation of the title compound was investigated in acetonitrile, dimethyl-sulphoxide and dichloroethane by means of cyclic voltammetry. The first electron transfer step is followed by the solvent substitution reaction in which the donor ability of the solvent plays an important role. The phenomenon of “curve crossing” was observed and was discussed in terms of solution electron transfer reactions involving the oxidation products and the parent species.

Redox series of complexes with a mixed coordination sphere: [Ru(bpy)2(5,5′-COOEtbphy)]2+

Abstract [Ru(bpy) 2 (5,5′-COOEtbphy)] 2+ is electrochemically reduced in six waves arranged into three doublets. The first doublet is ascribed on the basis of electrochemical, UV-visible spectral and ESR evidence, to the complete reduction of the hetero-ligand [Ru(bpy) 2 (5,5′-COOEtbpy 2− )] 0 being formed stepwise. The third electron is localized on the bpy 2 moiety. The fourth electron is most probably localized on bpy 2 as well; however, the ESR spectra did not show any signal. The last doublet at potentials of ∼ 3 V is electrochemically partially irreversible, but exhibits full chemical reversibility. The localization of the electron charge is also most probably on the bpy 2 moiety.

A semiempirical molecular orbital study of transition metals maleonitriledithiolate redox series

Abstract CNDO and iterative extended Huckel methods have been used for the study of the electronic structure of maleonitriledithiolate complexes of Co, Ni and Cu for various formal oxidation states of the central atom, the aim being to described the changes in the energy of molecular orbitals and in their composition depending on the overall number of electrons. The electronic charge distribution and bond orders connected with the variation of the overall charge of these complexes were also evaluated. The results of both methods were compared and used for the interpretation of redox properties. The electronic charge distribution reflects qualitatively the reactivity of sulphur centers. Both methods indicate, in the case of the redox series comnt n− 2 , remarkable orbital relaxation in accordance with experimental findings.

Electrochemistry of Fe(CO)2 (ν5-C5H5)X series. III. Mechanism of reduction for X = SiPh3, GePh3,SnPh3 — formation of anion radicals Fe(CO)2 (ν5-C5H5)MPh

Abstract Fe(CO) 2 (ν 5 -C 5 H 5 )MPh 3 (M = Si, Ge, Sn) compounds were shown to undergo a one electron reversible reduction resulting in the hitherto undescribed anion radicals Fe(CO) 2 (η 5 -C 5 H 5 )MPh . The radicals were characterized by their EPR spectra. The existence of these radicals is explained by the possibility of the interaction of the unpaired electron with the π-system of the phenyl groups system. These anion radicals undergo a further reduction which seems to be a concerted electron transfer and metal-metal bond rupture resulting in the formation of Fe(CO) 2 (η 5 -C 5 H 5 ) − and SnPh − 3 or HGePh 3 and HSiPh 3 , respectively.