Tris complexes reduction potentials

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

This order is always found for M = Mg, but not necessarily for M = Al, Cu, and Zn. In order to ensure that the required order is achieved, a complexing agent such as hexamethylphosphoramide (HMPA) or tris(3,6-dioxaheptyl)-amine (TDA-1) is added to render the metal reduction potential more negative. 

On the basis of the reduction potential of Rh(phen) (Eo = — 0.75 V/SCE) and of its nn energy (2.75 eV), Rh(phen)3 in the nn state is expected to be a very powerful oxidising agent (with a reduction potential of 2.0 V/SCE [133]), making it a stronger oxidant than the MLCT states of the Ru(II) complexes discussed above. Electron transfer from aromatic amines [134] or di-and tri-methoxybenzenes [135] to excited Rh(III) polypyridyl complexes have indeed been observed. 

A basic solution with a large excess of acetylacetone is strong blue. Complexes have been studied (log Pi = 5.383, log j82 = 10.189, log / 3 = 14.704). A potential of —1.0 V vs. SHE was measured for the reduction of the VUI tris complex.84 Mixed complexes V(L)2(py)2(L= acac, trifluoroacetylacetonato or dibenzoylmethanato) isolated from solutions containing VS04 and the appropriate ligand97 show intense absorption in the region 700-300 nm. 

Fig. 3. Correlation between the free energies of activation for the reactions given by Eq. (2) and the standard reduction potentials for the tris(diimine)metaKIII) complexes [cf. Eq. (4) ]. The data are from Table II.

Determinations of reduction potentials for a series of Fe(III) and Mn(III) tris-dithiocarbamate complexes by voltametry in the presence of various concentrations of polar molecules were conducted. The t y2 values varied linearly with the mole fraction of the particular polar molecule present (274). A theoretical model that was consistent with the experimentally derived f i/2 values was advanced. 

The earliest high-oxidation-state complex of nickel reported was the heteropoly(molybdate) (132, 133) complex [NilvMo90 32]6. which contains nickel(IV) in an octahedral Ni06 coordination environment. There is no evidence for the corresponding nickel(III) species but further work on nickel(IV) complexes of this type has been reported recently (134). Nickel(III) can be prepared in a six-coordinate oxygen donor environment (135) as a tris chelate with 2,2 -bipyridine-l,T-dioxide (bpy02). The complex has a rhombic EPR spectrum and a reduction potential of 1.7 V, from which an estimate of the reduction potential of the ion [Nini(H20)6]3+ of 2.5 V (versus nhe) has been calculated. 

A 10-step kinetic model has been developed (Santolini et al., 2001). Crystal structures of xyNOS show that a Tyr-409 indol nitrogen atom forms a strong hydrogen bond with the heme thiolate (Crane et al., 1988 Raman et al.1998 Fishmann et al., 1999). The Try-409 mutation suggests that the heme potential controls the NOS reactions (Adak et al. 2001). Suppression of this hydrogen bond through the mutation lowers the reduction potential of the heme, inhibits heme reduction and accelerates oxidation of the Fe(II) heme-NO complex. The Arg binding increases the reduction potential of the NOS heme. 

When M" is Mg ", this order is always observed but with Al, Zn, or Cu cations this is not always the case, and a metal deposition can subsequently occur at the cathode. To avoid this undesired reaction, we added a small quantity of a complexing agent like HMPA or tris(3,6-dioxaheptyl)amine (TDA-1) to the aprotic solvent, to shift the reduction potential of the cation towards more cathodic values (Fig. lb). 

The most comprehensive studies of the effect of acidity on potentials are those of Schilt 617, 618), where media up to 12 M in sulfuric acid were used. For M = Fe, Ru, or Os, the oxidation-reduction potential for M(bipy) +/M(bipy) + becomes less negative as the medium becomes more acid, while the converse is true for [M(bipy)2(CN)3]/[M(bipy)2(CN)2]. These results are interpreted as showing the formation of stable ion-pairs derived from the tris complexes and acid anions the CN groups in the mixed M(II) but not M(III) complexes may behave as bases yielding mono- and diprotonated species. 

If [Colenla] is taken as a reference complex for the tris-bidentate complexes, one observes that the unstrained complexes with five-membered rings have rather similar reduction potentials. The complexes of [Colpnlal , in acetone at least, become progressively easier to reduce with increasing ob content. The reduction potential of [Co(t-menlal is 450 mV more positive than [Co(en)3], indicating the destabilizing effect that axially oriented substituents have on the Co(III) complex. 

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