Reactivity of mononuclear rhodium(II) compounds

Mononuclear rhodium(II) compounds reported in the literature are tabulated, classified according to functionality and placed in one of four stability categories. The reactions reported for mononuclear Rh(II) compounds are presented and grouped according to reactivity type: adduct formation, ligand dissociation and substitution, dimerization, disproportionation, oxidation and reduction. Observed reactivity is rationalized on the basis of the factors that influence stability. Ligands that are sufficiently bulky can protect the metal from external attack by all but the smallest reagent molecules. Polydentate ligands can protect the metal from external attack or can impose coordination environments that destabilize Rh(I) or Rh(III). Electronic features of ligands that withdraw electron density or delocalize the unpaired electron from the metal promote stability. Reactivity can also depend on whether the Rh(II) complex is four coordinate (15 electron), five coordinate (17 electron) or six coordinate (19 electron). Because conventional two-electron oxidative addition and reductive elimination processes are not possible for Rh(II), redox behavior is of special interest. Mechanisms proposed for unusual oxidation and C-H activation behavior in some Rh(II) systems are presented. Factors giving rise to differences in reactivity between different classes of Rh(II) compounds are evaluated.