9.08 - Extreme Oxidation States of Transition Metals

Abstract

In this chapter we present a review of the quantum-chemical description of high and extreme oxidation states of transition metals. In the first part, the quantum-chemical methods used to describe these molecules are briefly presented. We also discuss the difficulties in assigning an oxidation state based on quantum‐chemical grounds. The highest possible oxidation state depends on the given element as well as on the used ligand system and highly electronegative ligands generally favor high‐oxidation numbers. The highest oxidation states are reached for the late 5d elements where the relativistic expansion of the d orbitals facilitates oxidation. The consideration of relativistic effects is thus crucial to correctly describe their high‐oxidation chemistry. Last but not least, some illustrating examples of highly oxidized transition metals are given. Oxidation states as high as VIII in OsO4 and IrO4 have been experimentally verified. Even the IX oxidation state has been predicted to be stable in the [IrO4]+ cation. In other cases the description of an unusually high‐oxidation state could be ruled out by quantum-chemical calculations, as for example in OHgF which only contains HgII and not HgIII. This leaves HgF4 as the only species where mercury is a true transition metal. Furthermore, other highly oxidized transition‐metal species like OsF7 and TcF7 have been predicted to be stable providing possible target molecules for future experimental attempts.