62
3 Technetium Coordinated by Inorganic Ligands in Aqueous …
and tetrabutyl perchlorate as supporting electrolytes. Presented data lead to a conclusion that the mobility of [Tc(NCS) 6 ]
3− ions is slightly higher than for [Fe(NCS) 6 ]
3− .
Further on, [TcCl 6 ]
2− ions are slightly more mobile than [Tc(NCS) 6 ]
2− .
Ammonium hexakis(isothiocyanato)technetate(IV) was also extensively studied
by Trop et al. (1980b). One-electron reversible [Tc(NCS) 6 ]
2−
/[Tc(NCS) 6 ]
3− couple
is characterized by a half wave with E 1/2 = 0.18 V versus SCE. Irreversible
reduction of [Tc(NCS) 6 ]
3− occurs at E 1/2 = −1.09 V. In turn, oxidation of
the [Tc(NCS) 6 ]
2− leads to formation of another one one-electron irreversible
wave with E 1/2 = 1.60 V. The value of the standard redox potential of the
[Tc(NO)(NCS) 5 ]
2−
/[Tc(NO)(NCS) 5 ]
3− couple is equal to 0.138 V versus SCE (see:
Clarke and Fackler 1982). The limiting conductance of the electrolytes containing
[NH 4 ] 2 [Tc(NCS) 6 ] (type 2:1) and [n−Bu 4 N] 3 [Tc(NCS) 6 ] (type 3:1) in acetonitrile
is equal to Λ 0 = 410 cm
2 ·
−1 ·equiv
−1 and 580 cm
2 ·
−1 ·equiv
−1 , respectively.
The chemistry of technetium and rhenium carbonyls and its halides are of special
interest in the context of its medical application as precursors for radiopharmaceuticals. However, discussion of the reactivity of these compounds goes beyond the
scope of this monograph and for more information on this topic the reader is referred
to the recent review papers (Alberto et al. 2004; Mazzi et al. 2007; Sidorenko et al.
2016).
Colton et al. (1960) analyzed electrochemical properties of rhenium carbonyl
and carbonyl halide complexes. Two electrons irreversible waves were observed for
all the systems analyzed when absolute ethanol containing 0.3 M methylammonium as a supporting electrolyte was used. Re
+ -CO system is presumably reduced
to the carbonyl rhenide anions. The electroreduction of Re(CO) 5 I (E 1/2 = −1.18 V)
and Re(CO) 5 Cl (E 1/2 = −1.27 V) is much easier as compared to Re 2 (CO) 10 (E 1/2
= −1.82 V). These authors observed that potentials of the half waves of rhenium
carbonyl halides decrease in the following order: Cl > Br > I. It can be assumed
with caution that the values of the appropriate potentials of technetium carbonyls or
technetium carbonyl-halides should be higher than for its rhenium analogs.
In conclusion, despite the huge number of publications that deal with studies on the
electroreduction of the pertechnetates and characterization of the reduced technetium
species in aqueous and nonaqueous solutions, the understanding of the electrochemical properties of technetium species is still incomplete. As an example, numerous
questions concern on the structure and disproportionation of Tc(VI) species present
in acidic and alkaline solutions. Further on, most recent reports on the stability of
the Tc(VI) are inconsistent with the earlier literature data. At the end of the twentieth century, the unstable Tc(V) was considered as generated exclusively in alkaline
solutions. Nowadays, it is well known that the Tc(V), although with a structure
different from the species existing at pH > 7, can be generated and stabilized also in
strongly acidic media. The mechanism of the Tc(IV) polymerization is also not fully
explained. Further on, the Tc(III) containing species await for much deeper exploration and for more detailed electrochemical description. It should be also stressed
that the structure of these ions is still unclear as well as the redox chemistry of polymeric Tc(IV) species. Finally, the generation of Tc(II) in aqueous noncomlpexing
media is a debatable issue. Therefore, it is very difficult to find reliable values of
3 Technetium Coordinated by Inorganic Ligands in Aqueous …
and tetrabutyl perchlorate as supporting electrolytes. Presented data lead to a conclusion that the mobility of [Tc(NCS) 6 ]
3− ions is slightly higher than for [Fe(NCS) 6 ]
3− .
Further on, [TcCl 6 ]
2− ions are slightly more mobile than [Tc(NCS) 6 ]
2− .
Ammonium hexakis(isothiocyanato)technetate(IV) was also extensively studied
by Trop et al. (1980b). One-electron reversible [Tc(NCS) 6 ]
2−
/[Tc(NCS) 6 ]
3− couple
is characterized by a half wave with E 1/2 = 0.18 V versus SCE. Irreversible
reduction of [Tc(NCS) 6 ]
3− occurs at E 1/2 = −1.09 V. In turn, oxidation of
the [Tc(NCS) 6 ]
2− leads to formation of another one one-electron irreversible
wave with E 1/2 = 1.60 V. The value of the standard redox potential of the
[Tc(NO)(NCS) 5 ]
2−
/[Tc(NO)(NCS) 5 ]
3− couple is equal to 0.138 V versus SCE (see:
Clarke and Fackler 1982). The limiting conductance of the electrolytes containing
[NH 4 ] 2 [Tc(NCS) 6 ] (type 2:1) and [n−Bu 4 N] 3 [Tc(NCS) 6 ] (type 3:1) in acetonitrile
is equal to Λ 0 = 410 cm
2 ·
−1 ·equiv
−1 and 580 cm
2 ·
−1 ·equiv
−1 , respectively.
The chemistry of technetium and rhenium carbonyls and its halides are of special
interest in the context of its medical application as precursors for radiopharmaceuticals. However, discussion of the reactivity of these compounds goes beyond the
scope of this monograph and for more information on this topic the reader is referred
to the recent review papers (Alberto et al. 2004; Mazzi et al. 2007; Sidorenko et al.
2016).
Colton et al. (1960) analyzed electrochemical properties of rhenium carbonyl
and carbonyl halide complexes. Two electrons irreversible waves were observed for
all the systems analyzed when absolute ethanol containing 0.3 M methylammonium as a supporting electrolyte was used. Re
+ -CO system is presumably reduced
to the carbonyl rhenide anions. The electroreduction of Re(CO) 5 I (E 1/2 = −1.18 V)
and Re(CO) 5 Cl (E 1/2 = −1.27 V) is much easier as compared to Re 2 (CO) 10 (E 1/2
= −1.82 V). These authors observed that potentials of the half waves of rhenium
carbonyl halides decrease in the following order: Cl > Br > I. It can be assumed
with caution that the values of the appropriate potentials of technetium carbonyls or
technetium carbonyl-halides should be higher than for its rhenium analogs.
In conclusion, despite the huge number of publications that deal with studies on the
electroreduction of the pertechnetates and characterization of the reduced technetium
species in aqueous and nonaqueous solutions, the understanding of the electrochemical properties of technetium species is still incomplete. As an example, numerous
questions concern on the structure and disproportionation of Tc(VI) species present
in acidic and alkaline solutions. Further on, most recent reports on the stability of
the Tc(VI) are inconsistent with the earlier literature data. At the end of the twentieth century, the unstable Tc(V) was considered as generated exclusively in alkaline
solutions. Nowadays, it is well known that the Tc(V), although with a structure
different from the species existing at pH > 7, can be generated and stabilized also in
strongly acidic media. The mechanism of the Tc(IV) polymerization is also not fully
explained. Further on, the Tc(III) containing species await for much deeper exploration and for more detailed electrochemical description. It should be also stressed
that the structure of these ions is still unclear as well as the redox chemistry of polymeric Tc(IV) species. Finally, the generation of Tc(II) in aqueous noncomlpexing
media is a debatable issue. Therefore, it is very difficult to find reliable values of