Technetium and rhenium alkoxides

This section is actually devoted to the description of rhenium alkoxides, as the technetium ones are rarely studied. The latter are reviewed in a short appendix at the end. The chemistry ofrhenium in lower oxidation states is much alike that of ruthenium and therefore even for rhenium, the lower oxidation state complexes with jt-acceptor ligands are described in this chapter. 

The main route to rhenium alkoxides is the interaction of halids and oxy-halids with alkali alkoxides or alcohols in presence of amines (method 5). As the important starting reagents can serve also Re2(CO),0 and Re207 (method 3) The preparation ofrhenium (V) and (VI) oxoderivatives by the anodic oxidation of metal in alcohols has also been described (method 2) (see Table 12.22). The bimetallic alkoxides ofrhenium and heavy transition metals can most efficiently be obtained by interaction ofrhenium (VII) oxide with the alkoxides ofthese elements in refluxing toluene  

The chemistry of the alkoxoderivatives of rhenium is very diversified due to the existence of the compounds corresponding to practically all the possible oxidation states. The transitions between them occur usually quite easily, being characterized by comparably low values of redox potentials as it takes place, for example, for rhenium (VI) cateholates [471]  

The observed E0 values in this case indicate the highest stability of rhenium (V) derivatives in this series. The comparison of the preparative data published indicates that these are the +5 and +6 oxidation states that appear to be most stable for the alkoxocomplexes of rhenium. The low-valent (+1 — +3) complexes should either be stabilized by JT-acceptor ligands (CO, PR3, NO, unsaturated hydrocarbons) or contain multiple M M bonds [321, 586, 729, 762,]. The compounds of rhenium (VII) are very unstable and decompose at room temperature in several minutes when isolated. They can be isolated and kept for several days as the complexes with N-donor ligands such as Tmeda or Py [533, 519, 1358]. The decomposition products of rhenium (VI) and (VIT) alkoxides are often described in literature as a black tar. The compound with this kind of appearence turned to be the major product of the anodic oxidation of rhenium in methanol (at high current density) and was shown by the X-ray single crystal study to be Re402(0Me)16 [906]. 

The other feature that enriches but also complicates the chemistry of rhenium alkoxides is their ability to use different kinds of decomposition processes, not leading to the changes in the oxidation state. That is first and foremost the ether elimination from the rhenium (V-VII) derivatives. It leads to the formation of oxocomplexes, as, for example [168]  

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