Halides, oxidation-reduction behavior

Substituted complexes of the type [M (N,N)2XY]" M = Ru, Os, X,Y = halides, CN , 204 , py, en, NH3, etc. show, in general, two doublets of N,N-localized reductions [154-156]. The reduction behavior is complicated by loss of an ancillary ligand X upon the second and, more rapidly, the 3rd bpy-localized reduction. The reduction potentials are only slightly X-dependent. On the other hand, the oxidation potential is highly dependent on X. Extensive tables of oxidation and the first reduction potentials of these complexes are available [15, 28, 74, 157]. Their values can be predicted by use of electrochemical ligand parameters [15, 28, 157]. 

The indirect electrochemical reduction of alkyl halides is also possible by use of nickel(I) complexes which may be obtained by cathodic reduction of square planar Ni(n)-complexes of macrocyclic tetradentate ligands (Table 7, No. 10, 11) 2 4-248) Comparable to the Co(I)- and Ni(O)-complexes, the Ni(I)-species reacts with the alkyl halide unter oxidative addition to form an organo nickel(III) compound. The stability of the new nickel-carbon bond dominates the overall behavior of the system. If the stability is low, the alkyl group is lost in form of the radical and the original Ni(II)-complex is regenerated. A large number of regenerative cycles is the result. 

The redox behavior of complexes of the type [Re (X)(CO)3(R-dab)] X = halide is similar to that of their polypyridine counterparts—one oxidation and two successive reductions which ultimately produce pentacoordinated [Re (CO)3(R-dab)] species [134, 136, 142]. There are, however, some remarkable differences between the redox chemistry of Re dab and polypyridine complexes (i) Oxidation [134] to [Re (X)(CO)3(R-dab)]+ is reversible and less positive (-1-0.58 V for X = Cl, R = tBu), indicating the greater stability of Re Cdab complexes and smaller involvement of E ligand orbitals in the HOMO of the Re complex, (ii) The rate of dissociation of the axial ligand after the first reduction to [Re (Br)(CO)3(R-dab )] ... 

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