Taube reactivity

Two studies have been made of the reductions of NH2OH and its analogues by Cr(ri) perchlorate which dilfer seriously both as regards the results and their interpretation. The results of both studies are combined in Table 22. Wells and Salam find NH2OH and N2H4 to be much more reactive towards Cr(II) than do Taube et (who find no reaction with N2H4 in 24 h at 25 °C). 

DR. TAUBE A problem with the molybdenum catalysis is that the species which reacts readily with CIO may be an unstable, possibly mononuclear, species rather than one of the condensed forms which we ordinarily encounter. Paffett and Anson [Paffett, M. T. Anson, F. C. Inorg. Chem. 1981 20, 3967] have shown that mononuclear Mo(V) reacts moderately rapidly with C10. My guess is that mononuclear Mo(IV) would be even more reactive. Mononuclear Mo(IV) is probably a typical yl ... 

As discussed in Vol. 2, Chap. 4, experimental studies, mainly pioneered by Taube [11], revealed two different reaction pathways for redox reactions in solution (i) outer sphere mechanism characterized by weak interaction of the reactive species, with the inner coordination sphere remaining intact during the electron transfer, and reactions occurring through a common ligand shared by the metallic centers thus proceeding by an inner sphere mechanism. 

In contrast to stability, lability and inertness are kinetic quantities. A labile (or robust) complex is quite reactive an inert entity is quite slow to react. A criterion has been suggested by Taube (41) "labile is applied to those systems where reactions are complete within time of mixing—i.e., within 1 minute at room temperature "inert is applied to systems in which reactions are too slow to measure or occur at measurable (by conventional techniques) rates. 

Taube and co-workers (117) investigated the synthesis and reactivities of [MIV(Cp)2X]+ (M = Ru, Os X = halide). These species behave as two-electron atom transfer donors and the self-exchange rate constants of the couples [M(Cp)2X]+ /[M(Cp)2] have been determined by H NMR measurements. 

Another topic of great interest in the 1950s concerned the mechanism of transfer of electrons between transition metal ions in solution. Work on this topic earned H. Taube a Nobel prize in 1983. Prior to this, the 1981 prize was awarded to K. Fukui and R. Hoffmann for their work on theoretical models of bonding and reactivity, which included studies of transition element compounds. The ability of transition metals to bond to one another directly has provided another active area of research. This has provided examples of metal clusters containing from two up to hundreds of metal atoms linked by metal-metal bonds. Chemists can now investigate the point at which a group of metal atoms becomes sufficiently small so that it ceases to behave like a metallic material and assumes the properties of a molecular entity. 

Because the HO-OH bond in hydrogen peroxide is weak (AHdbE/ 51 versus 90 kcal for the H-OOH bond), there has been a tendency to assume homolytic cleavage in the reactivity and activation of HOOH. However, in a pioneering study of the conversion by HOOH of sulfur dioxide to sulfuric acid Halperin and Taube 2 proposed a nucleophilic-addition mechanism. 

Ruthenium(ii) ammines. Reactivity, H. Taube, Surv. Progr. Chem., 1973,6,1. 

More important, the failure of many transition metal aqua ions to fit the correlations of Figure 8.4 highlights the influence of d electron configuration on the reactivity of metal aqua ions in substitution reactions. The importance of d electron configuration was first noted by Taube in 19521 and explained qualitatively in terms of valence bond theory. Taube, with his predilection for simple test tube demonstrations, distinguished labile metal complexes (ones which underwent substitution within the time of mixing) from inert ones, the latter being typically octahedral complexes... 

From the work of Taube and King, it was suspected that the different reactivities of these presumably identical compounds were caused by catalytic amounts of [Pt(en)2]. This was confirmed both by the addition of the catalyst and also by the addition of cerium(iv) to destroy the catalyst. Addition of five mole per cent [Pt(en)2] results in complete reaction within five minutes of a complex which would otherwise react in the dark at 25 °C with = 44 min. 

Taube was the first to offer an explanation for the variable lability of these metal ions by classifying them qualitatively on the basis of their reactivity. 

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