4.1 Nonaqueous Solutions
73
of Tc(III)/Tc(II) redox couples and quasi-reversible one-electron Tc(IV)/Tc(III)
systems. Although ligands with higher acidity should stronger stabilize the complex
due to favored delocalization of the charge density from the Tc(II), the reduction of
[Tc(L
1 ) 3 ] complex with the strongest π-acid ligand (phosphino-benzoic) takes place
at potentials lower than for the other complexes. In this case, steric factors are so
important that they change redox properties of the couple significantly.
In 1991, Wilcox with Deutsch (1991) published an article that was a continuation of earlier works on Tc complexes with derivatives of phosphines as ligands.
The research focuses on the redox properties of Tc(III/II) complexes containing
dimethyl(or diethyl)phenylphosphines and polypyridyl as ligands. Also this time,
the reactions of Tc(IV)/Tc(III) and Tc(III)/Tc(II) redox couples containing (bypy)
turned out to be diffusion controlled one-electron processes. (Me 2 bpy) stabilizes
higher oxidation states of Tc, e.g., Tc(IV) more effectively than (bpy) ligands because
the former ligand is a better σ donor and poorer π acceptor than the latter. Moreover,
Tc(IV) complexes are generally less stable than corresponding Tc(II) containing
species. Early work of Breikss et al. (1990) deals with electrochemical studies on
properties of Tc(III) coordinated by mixed phosphine, chloride and nitrogen-donors.
Also for these systems, a reversible Tc(IV)/Tc(III) couple was observed at potentials
close to those reported by Wilcox and Deutsch (1991).
An increase in the number of phenyl groups in phosphine ligands facilitates the
electroreduction of Tc(III) to Tc(II) but impedes electrooxidation of Tc(III) to Tc(IV).
The ligands with Ph- groups are stronger π-acids and more efficiently stabilize
Tc(II) in opposite to alkyl-substituted ligands, which are more effective σ-donors
and better stabilize Tc(IV). The halogens are the other ligands able to stabilize the
lower oxidation states of Tc and this effect is somewhat stronger for the heavier
halogens.
Konno et al. (1988, 1989a, b, 1992a, b, 1993) studied electrochemistry of Tc
complexes containing phosphines and thiolato ligands. The results indicate a oneelectron reversible reduction of Tc(III) to Tc(II). The redox properties of Tc(II)/Tc(I)
and Tc(IV)/Tc(III) strongly depend on the ligand properties. For example (depe),
which is a σ-donor better than (dmpe), provides a stronger stabilization of the higher
Tc oxidation states. Moreover, replacement of the halogens with stronger σ-donating
thiolato ligands in Tc(III) complexes shifts the E
0 values by several hundred mV
toward more negative ones. Aromatic substituents in such types of ligands enhance
their π-acid properties and this results in E
0 values more positive than for alkyl
substitutes. The E
0 of examined Tc-complexes decreases according to changes of
thiolato ligands structures in the following order: SC 6 H 4 -p-Cl > SC 6 H 5 > SC 6 H 4 -
p-CH 3 > SCH 2 C 6 H 5 > SCH 2 C 6 H 4 -p-OCH 3 > SC 2 H 5. An increase in the length of
the alkyl group in the ligand makes the Tc(III) complex more difficult to reduce but
easier to oxidize.
Konno et al. (e.g. 1989b) discussed the influence of the spatial distribution of
the ligands in a complex on the redox properties of the latter. The steric repulsion
between various groups as well as ligand competition for the electron density of
Tc center are among the most important factors that determine the stability of Tc
complexes. For example, cis-geometry in [Tc(SC 6 H 5 ) 2 (diars) 2 ] makes its reduction
73
of Tc(III)/Tc(II) redox couples and quasi-reversible one-electron Tc(IV)/Tc(III)
systems. Although ligands with higher acidity should stronger stabilize the complex
due to favored delocalization of the charge density from the Tc(II), the reduction of
[Tc(L
1 ) 3 ] complex with the strongest π-acid ligand (phosphino-benzoic) takes place
at potentials lower than for the other complexes. In this case, steric factors are so
important that they change redox properties of the couple significantly.
In 1991, Wilcox with Deutsch (1991) published an article that was a continuation of earlier works on Tc complexes with derivatives of phosphines as ligands.
The research focuses on the redox properties of Tc(III/II) complexes containing
dimethyl(or diethyl)phenylphosphines and polypyridyl as ligands. Also this time,
the reactions of Tc(IV)/Tc(III) and Tc(III)/Tc(II) redox couples containing (bypy)
turned out to be diffusion controlled one-electron processes. (Me 2 bpy) stabilizes
higher oxidation states of Tc, e.g., Tc(IV) more effectively than (bpy) ligands because
the former ligand is a better σ donor and poorer π acceptor than the latter. Moreover,
Tc(IV) complexes are generally less stable than corresponding Tc(II) containing
species. Early work of Breikss et al. (1990) deals with electrochemical studies on
properties of Tc(III) coordinated by mixed phosphine, chloride and nitrogen-donors.
Also for these systems, a reversible Tc(IV)/Tc(III) couple was observed at potentials
close to those reported by Wilcox and Deutsch (1991).
An increase in the number of phenyl groups in phosphine ligands facilitates the
electroreduction of Tc(III) to Tc(II) but impedes electrooxidation of Tc(III) to Tc(IV).
The ligands with Ph- groups are stronger π-acids and more efficiently stabilize
Tc(II) in opposite to alkyl-substituted ligands, which are more effective σ-donors
and better stabilize Tc(IV). The halogens are the other ligands able to stabilize the
lower oxidation states of Tc and this effect is somewhat stronger for the heavier
halogens.
Konno et al. (1988, 1989a, b, 1992a, b, 1993) studied electrochemistry of Tc
complexes containing phosphines and thiolato ligands. The results indicate a oneelectron reversible reduction of Tc(III) to Tc(II). The redox properties of Tc(II)/Tc(I)
and Tc(IV)/Tc(III) strongly depend on the ligand properties. For example (depe),
which is a σ-donor better than (dmpe), provides a stronger stabilization of the higher
Tc oxidation states. Moreover, replacement of the halogens with stronger σ-donating
thiolato ligands in Tc(III) complexes shifts the E
0 values by several hundred mV
toward more negative ones. Aromatic substituents in such types of ligands enhance
their π-acid properties and this results in E
0 values more positive than for alkyl
substitutes. The E
0 of examined Tc-complexes decreases according to changes of
thiolato ligands structures in the following order: SC 6 H 4 -p-Cl > SC 6 H 5 > SC 6 H 4 -
p-CH 3 > SCH 2 C 6 H 5 > SCH 2 C 6 H 4 -p-OCH 3 > SC 2 H 5. An increase in the length of
the alkyl group in the ligand makes the Tc(III) complex more difficult to reduce but
easier to oxidize.
Konno et al. (e.g. 1989b) discussed the influence of the spatial distribution of
the ligands in a complex on the redox properties of the latter. The steric repulsion
between various groups as well as ligand competition for the electron density of
Tc center are among the most important factors that determine the stability of Tc
complexes. For example, cis-geometry in [Tc(SC 6 H 5 ) 2 (diars) 2 ] makes its reduction