Figure 2.

Qualitative frontier molecular orbital splitting diagrams for (a) an octahedral metal complex and a metal oxo residing in a (b) tetragonal field, (c) trigonal bipyramidal and (d) tetrahedral ligand field. The table shows the d-electron count that supports a multiple metal oxo bond and the preferred mechanism for the O–O bond coupling, where AB is acid/base and RC, radical coupling. Only low-spin configurations are considered. The assignment of AB and RC will change with high-spin configurations.