The anodic electrochemical oxidations of ReCp(CO)
3 (
1, Cp =
5-C
5H
5), Re(
5-C
5H
4NH
2)(CO)
3(
2), and ReCp*(CO)
3 (
3, Cp* =
5-C
5Me
5), have been studied in CH
2Cl
2 containing [NBu
4][TFAB] (TFAB =[B(C
6F
5)
4]
-) as supporting electrolyte. One-electron oxidations were observed with
E1/2 = 1.16, 0.79, and0.91 V vs ferrocene for
1-
3, respectively. In each case, rapid dimerization of the r
adical cation gave thedimer dication, [Re
2Cp
2(CO)
6]
2+ (where Cp
![](/images/gifchars/gamma.gif)
represents a generic cyclopent
adienyl ligand), which may beitself reduced cathodically back to the original 18-electron neutral complex ReCp
![](/images/gifchars/gamma.gif)
(CO)
3. DFT calculationsshow that the SOMO of
1+ is highly Re-based and hybridized to point away from the metal, thereby facilitatingthe dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases,have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculatedas 3.229 Å for [Re
2Cp
2(CO)
6]
2+ (
122+) and measured as 3.1097 Å for [Re
2(C
5H
4NH
2)
2(CO)
6]
2+ (
222+) byX-ray crystallography on [Re
2(C
5H
4NH
2)
2(CO)
6][TFAB]
2. The monomer/dimer equilibrium constants arebetween
Kdim = 10
5 M
-1 and 10
7 M
-1 for these systems, so that partial dissociation of the dimers gives amodest amount of the corresponding monomer that is free to undergo r
adical cation reactions. The r
adical
1+ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO)
3Cl]
+as a side product. The r
adical cation
1+ acts as a powerful one-electron oxidant capable of effectivelydriving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.