Several metal ions of oxidation state 3 + form substitutioninert octahedral complexes: the d3 species Cr(III) and Mo(III); the low spin d5 species Ru(III) and Os(III); the low spin d6 species Co(III), Rh(III), Ir(III). Because of the advantages which slow substitution reactions offer in being amenable to searching tests of mechanism, these 3 + centers have been the subject of much study. Metal ions in the 2+ oxidation state which form substitution-inert octahedral complexes are much less common, and in fact Ru(II) is unique in being the only one which forms a series that is both abundant and convenient for study. It is for this reason in part that the Ru(II) complexes provide the main focus for this article. There are other important reasons as well.