ABSTRACT
A lower valent and nucleophilic oxidation state such as Co(I) can pick up a proton or methyl cation as “ ligand’ ’ to form a product whose d-d spectrum shows that it is best considered as a Co(III) complex with a hydride or carbanion as ligand (Chapter 13, Sec. 1.5), i.e., in this case the incoming reagent is a net (formally two-electron) accep tor. One might then expect to find a correlation between log K for the binding of a given acceptor and the “ basicity” or reducing potential of a family of related metal ions. Good linear correlations have indeed been observed between log K for the bind ing of 0 2 as ligand to several families of macrocyclic Co(II) complexes and the poten tial (varied by changing the trans ligand) for oxidation to Co(III) [46], and a more general correlation between log K for the binding of 0 2 by the Fe(II) ion in a range of Hb’s and Mb’s and the potential for oxidation to Fe(III) [47]. The ESR spectra of the complexes derived from Co(II) and 0 2 indicate that their formulation is closer to Co(III) complexes of the superoxide radical anion and demonstrate that coordinated 0 2 is a net electron acceptor [48] and not a donor, as sometimes suggested in the literature. Nitroso compounds RNO, formally isoelectronic with 0 2 although diamag netic, also react with both low-spin Fe(II) and Co(II) complexes, and the ESR spectra of the latter again indicate that their formulation is closer to Co(III) complexes of the RNO“ radical anion [49]. Such a partial transfer of charge increases the positive charge on the metal and the negative charge on the ligand atom and thereby increases the coulombic contribution to the M-L bond energy. It may occur with reducing ions such as Fe(II), Co(II), and Cu(I), but scarcely with Ni(II); the very strongly reducing Co(I) may simply reduce the reagent without forming any detectable intermediate complex.
