Rhenium substitution reactions

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

The results presented in Section IV.D.l show that the net loss of the dioxorhenium(VII) species, and the ultimate formation of the phosphate R3PO, must occur in two stages because the rate of reaction (17) shows a direct first-order phosphine dependence. That said, the chemical mechanism is still open to discussion does the first step entail abstraction of an oxo oxygen or addition to it If the former, the cycle is completed by PR3 coordination to a four-coordinate rhenium intermediate if the latter, the addition step is then followed by yet another ligand substitution reaction. The alternatives are presented in Schemes 1 and 2. 

Scheme 6. Turnstile mechanism for the ligand substitution reactions of oxo-rhenium(V) compounds.

The work cited in sections 2.4 and 2.5 is representative of the SN1 substitution reactions of metal carbonyls. However, a much more extensive and detailed account has recently been published covering similar reactions of vanadium, chromium, molybdenum, tungsten, rhenium, iron and nickel carbonyls in addition to those of manganese and cobalt2 9a. 

In the case of technetum, this is the most practically used element among non-/ radioactive ones for medical and technical purposes [283], so the permanent interest in its coordination chemistry (in particular, the structural aspect of its compounds [547] and kinetics of substitution reactions [548]) is not surprising [549]. The theoretical interest in Tc is provoked, in particular, by the fact that this is a rhenium analogue. This element (Re) forms multiple metal-metal bond complexes and has been studied intensively in order to achieve a better understanding of the physical and chemical properties of multiple bonds between metal atoms [533],... 

Carbonyl halides of chromium, Cr(CO)5I and others (71), as well as Mo(CO)4C12 (72), have only recently been added to the list of known carbonyl halides. Moreover, the substitution reactions of the carbonyl halides of manganese and rhenium (73) as well as those of the noble metals (74) have been thoroughly studied. A comprehensive review on this topic has recently been published.4... 

An example of a chemically induced substitution reaction is represented by the halide removal in MnCl(CO)5 by AICI3 leading to the hexacarbonylmanganese(I) cation. The driving force of this reaction, carried ont at room temperature, is represented by the high affinity of aluminum for chloride. Similarly, the corresponding rhenium(I) derivative [Re(CO)6]AlCLi can be prepared. This complex has considerable stability in that it can be dissolved in water without prompt decomposition (equation 32). 

Hi) Rhenium(V) complex with NCS (191), thiourea, and other incoming ligands (205). The observed linear free-energy relationship also suggests dissociative activation for the substitution reactions of the rhenium complex (205). 

Rhenium(V) complexes (and probably rhenium complexes in other oxidation states) are more substitution-inert dian their technetium analogs [8,9]. This has direct consequences for the synthesis of rhenium radiopharmaceuticals via ligand substitution reactions [2] such syntheses are very important in the labeling of monoclonal antibodies with radiorhenium [4,5]. 

---