Electronic Structures of Oxo-Metal Ions

The dianionic oxo ligand occupies a very special place in coordination chemistry, owing to its ability to donate π electrons to stabilize high oxidation states of metals. The ligand field theory of multiple bonding in oxo-metal ions, which was formulated in Copenhagen 50 years ago, predicts that there must be an “oxo wall” between Fe–Ru–Os and Co–Rh–Ir in the periodic table. In this tribute to Carl Ballhausen, we review this early work as well as new developments in the field. In particular, we discuss the electronic structures of beyond-the-wall (groups 9 and 10) complexes containing metals multiply bonded to O- and N-donor ligands.