Metal Complexes with Reducible Ligands

Strong evidence for the feasibility of the chemical mechanism (Sec. 5.8.1) has been afforded by the production of a transient nitrophenyl radical attached to a Co(III) complex 

This is indicated by the observation of first-order loss of radical signal, with the rate constant 1 being independent of the concentration, both of radical complex (i. e., independent of the dose of reductant radical used) and of Co —L complex used (in excess). Only rarely is the first-order loss of Co —L directly dependent on the concentration of Co —L present. 

In all cases (5.79)-(5.81), the cobalt(ll) product is expected to rapidly dissociate to constituents. The presence of (5.80) makes it difficult to determine the value of, which is a small intercept on a k vs [Co — L] plot. The occurrence of (5.81) is usually signalled by mixed first- and second-order kinetics since as the concentration of Co —L- (or Co —L H) decreases, the importance of (5.81) is superseded by (5.79) and k results from the (first-order) end of the decay (see Fig. 1.6). Most unusually, the complex (NH3)5Co(7V-Mebpy ) undergoes all three modes of decay (Table 5.11). 

There has been nothing like the enthusiasm for the application to these systems of the theoretical equations, which we have noted in the previous sections and will encounter in the next. Nevertheless, a number of features are present which are qualitatively consistent with the discussions in Sec. 5.8.1 and which are in part illustrated in Table 5.11. There is a correlation of rate constant with the driving force of the internal electron transfer. -pjjg p. itro-phenyl derivative is a poorer reducing agent when protonated and k is much less than for the unprotonated derivative. Consequently disproportionation (2A 3) becomes important. Although there are not marked effects of structural variation on the values of A , the associated activation parameters may differ enormously and this is ascribed to the operation of different mechanisms. The resonance-assisted through-chain operates with the p- 

Finally, attention should be drawn to the elegant studies of Meyer and his co-workers. Metal complexes have been designed which contain both an excitable (e. g. bpy) and a quencher (e. g. Af-Mebpy+) group. Following excitation, intramolecular electron and energy transfer occurs and the dependence of rate on distances, metal and so on, can be assessed. 

---

The aquated iron(III) ion is an oxidant. Reaction with reducing ligands probably proceeds through complexing. Rapid scan spectrophotometry of the Fe(III)-cysteine system shows a transient blue Fe(lII)-cysteine complex and formation of Fe(II) and cystine. The reduction of Fe(lII) by hydroquinone, in concentrated solution has been probed by stopped-flow linked to x-ray absorption spectrometry. The changing charge on the iron is thereby assessed. In the reaction of Fe(III) with a number of reducing transition metal ions M in acid, the rate law... 

That this comparison seems to have validity (neglecting the legitimated analogy between bis-JT-allylnickel and symmetry-reduced benzene) may be atMboted to the fact that open-chain TT-systems may display some residual aromaticity (see Ref. ). Metal complexes with C3 symmetry must have three identical ligands (S) in the same state. When we change only one l and (S versus L) symmetry is reduced. The structural consequence is the change to a T- or Y-shaped moiety (see Fig. 1 in Scheme 2.5-4 and similar discussions in Refs. Within thcK arran ... 

In solvents, the complexes of metal ions with organic ligands are more soluble, which makes studies easier moreover, in anhydrous and aprotic media, it is possible to reduce the catalysts and their oxygen complexes out of reach of protons and thus to study the first steps of the catalytic cycle. It has been possible to show by studies in benzonitrile that the mixed-valence Co Co redox state of the... 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

A secondary, more subtle, effect that can be utilized in the achievement of selectivity in cation exchange is the selective complexation of certain metal ions with anionic ligands. This reduces the net positive charge of those ions and decreases their extraction by the resin. In certain instances, where stable anionic complexes form, extraction is suppressed completely. This technique has been utilized in the separation of cobalt and nickel from iron, by masking of the iron as a neutral or anionic complex with citrate350 or tartrate.351 Similarly, a high chloride concentration would complex the cobalt and the iron as anionic complexes and allow nickel, which does not form anionic chloro complexes, to be extracted selectively by a cation-exchange resin. 

Nowadays, not only Fe but other trace metals as well, for example, Mn, Co, or Cu, are thought to limit primary production. It is thus a real challenge for oceanographers not just to assess correctly the very low levels of Fe and Mn in the oceans but also to carry out the speciation of these elements (total dissolved concentrations are at the nM level, labile forms oxidation states in natural aquatic systems Fe(II), which is readily soluble, and Fe(III), which is almost insoluble. Flowever, both Fe ions can form diverse complexes with organic ligands with different labilities and solubilities, and colloidal particles, which are also considered part of the dissolved phase. Manganese also exists in two oxidation states in aquatic systems soluble Mn(II) and insoluble Mn(IV) both are present in a dynamic cycle in seawater. The nonlabile Mn pool consists of oxidized Mn(IV) species, but these can be photochemically reduced and thus solubilized.23... 

---